diff --git a/src/compiler/binder.ts b/src/compiler/binder.ts
index 35a62a644d839..467c25f549e36 100644
--- a/src/compiler/binder.ts
+++ b/src/compiler/binder.ts
@@ -1550,7 +1550,7 @@ namespace ts {
         function setExportContextFlag(node: ModuleDeclaration | SourceFile) {
             // A declaration source file or ambient module declaration that contains no export declarations (but possibly regular
             // declarations with export modifiers) is an export context in which declarations are implicitly exported.
-            if (isInAmbientContext(node) && !hasExportDeclarations(node)) {
+            if (node.flags & NodeFlags.Ambient && !hasExportDeclarations(node)) {
                 node.flags |= NodeFlags.ExportContext;
             }
             else {
@@ -1726,7 +1726,7 @@ namespace ts {
                 node.originalKeywordKind >= SyntaxKind.FirstFutureReservedWord &&
                 node.originalKeywordKind <= SyntaxKind.LastFutureReservedWord &&
                 !isIdentifierName(node) &&
-                !isInAmbientContext(node)) {
+                !(node.flags & NodeFlags.Ambient)) {
 
                 // Report error only if there are no parse errors in file
                 if (!file.parseDiagnostics.length) {
@@ -2481,7 +2481,7 @@ namespace ts {
         }
 
         function bindParameter(node: ParameterDeclaration) {
-            if (inStrictMode && !isInAmbientContext(node)) {
+            if (inStrictMode && !(node.flags & NodeFlags.Ambient)) {
                 // It is a SyntaxError if the identifier eval or arguments appears within a FormalParameterList of a
                 // strict mode FunctionLikeDeclaration or FunctionExpression(13.1)
                 checkStrictModeEvalOrArguments(node, node.name);
@@ -2503,7 +2503,7 @@ namespace ts {
         }
 
         function bindFunctionDeclaration(node: FunctionDeclaration) {
-            if (!file.isDeclarationFile && !isInAmbientContext(node)) {
+            if (!file.isDeclarationFile && !(node.flags & NodeFlags.Ambient)) {
                 if (isAsyncFunction(node)) {
                     emitFlags |= NodeFlags.HasAsyncFunctions;
                 }
@@ -2520,7 +2520,7 @@ namespace ts {
         }
 
         function bindFunctionExpression(node: FunctionExpression) {
-            if (!file.isDeclarationFile && !isInAmbientContext(node)) {
+            if (!file.isDeclarationFile && !(node.flags & NodeFlags.Ambient)) {
                 if (isAsyncFunction(node)) {
                     emitFlags |= NodeFlags.HasAsyncFunctions;
                 }
@@ -2534,7 +2534,7 @@ namespace ts {
         }
 
         function bindPropertyOrMethodOrAccessor(node: Declaration, symbolFlags: SymbolFlags, symbolExcludes: SymbolFlags) {
-            if (!file.isDeclarationFile && !isInAmbientContext(node) && isAsyncFunction(node)) {
+            if (!file.isDeclarationFile && !(node.flags & NodeFlags.Ambient) && isAsyncFunction(node)) {
                 emitFlags |= NodeFlags.HasAsyncFunctions;
             }
 
@@ -2583,7 +2583,7 @@ namespace ts {
                     //   On the other side we do want to report errors on non-initialized 'lets' because of TDZ
                     const reportUnreachableCode =
                         !options.allowUnreachableCode &&
-                        !isInAmbientContext(node) &&
+                        !(node.flags & NodeFlags.Ambient) &&
                         (
                             node.kind !== SyntaxKind.VariableStatement ||
                             getCombinedNodeFlags((<VariableStatement>node).declarationList) & NodeFlags.BlockScoped ||
diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 72438fa27af87..5e56fe6295f9b 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -691,7 +691,7 @@ namespace ts {
             else {
                 // find a module that about to be augmented
                 // do not validate names of augmentations that are defined in ambient context
-                const moduleNotFoundError = !isInAmbientContext(moduleName.parent.parent)
+                const moduleNotFoundError = !(moduleName.parent.parent.flags & NodeFlags.Ambient)
                     ? Diagnostics.Invalid_module_name_in_augmentation_module_0_cannot_be_found
                     : undefined;
                 let mainModule = resolveExternalModuleNameWorker(moduleName, moduleName, moduleNotFoundError, /*isForAugmentation*/ true);
@@ -796,7 +796,7 @@ namespace ts {
                 if ((modulekind && (declarationFile.externalModuleIndicator || useFile.externalModuleIndicator)) ||
                     (!compilerOptions.outFile && !compilerOptions.out) ||
                     isInTypeQuery(usage) ||
-                    isInAmbientContext(declaration)) {
+                    declaration.flags & NodeFlags.Ambient) {
                     // nodes are in different files and order cannot be determined
                     return true;
                 }
@@ -1384,7 +1384,7 @@ namespace ts {
 
             Debug.assert(declaration !== undefined, "Declaration to checkResolvedBlockScopedVariable is undefined");
 
-            if (!isInAmbientContext(declaration) && !isBlockScopedNameDeclaredBeforeUse(declaration, errorLocation)) {
+            if (!(declaration.flags & NodeFlags.Ambient) && !isBlockScopedNameDeclaredBeforeUse(declaration, errorLocation)) {
                 if (result.flags & SymbolFlags.BlockScopedVariable) {
                     error(errorLocation, Diagnostics.Block_scoped_variable_0_used_before_its_declaration, declarationNameToString(getNameOfDeclaration(declaration)));
                 }
@@ -3940,7 +3940,7 @@ namespace ts {
                         const parent = getDeclarationContainer(node);
                         // If the node is not exported or it is not ambient module element (except import declaration)
                         if (!(getCombinedModifierFlags(node) & ModifierFlags.Export) &&
-                            !(node.kind !== SyntaxKind.ImportEqualsDeclaration && parent.kind !== SyntaxKind.SourceFile && isInAmbientContext(parent))) {
+                            !(node.kind !== SyntaxKind.ImportEqualsDeclaration && parent.kind !== SyntaxKind.SourceFile && parent.flags & NodeFlags.Ambient)) {
                             return isGlobalSourceFile(parent);
                         }
                         // Exported members/ambient module elements (exception import declaration) are visible if parent is visible
@@ -4318,7 +4318,7 @@ namespace ts {
 
             if ((noImplicitAny || isInJavaScriptFile(declaration)) &&
                 declaration.kind === SyntaxKind.VariableDeclaration && !isBindingPattern(declaration.name) &&
-                !(getCombinedModifierFlags(declaration) & ModifierFlags.Export) && !isInAmbientContext(declaration)) {
+                !(getCombinedModifierFlags(declaration) & ModifierFlags.Export) && !(declaration.flags & NodeFlags.Ambient)) {
                 // If --noImplicitAny is on or the declaration is in a Javascript file,
                 // use control flow tracked 'any' type for non-ambient, non-exported var or let variables with no
                 // initializer or a 'null' or 'undefined' initializer.
@@ -5191,7 +5191,7 @@ namespace ts {
         function isLiteralEnumMember(member: EnumMember) {
             const expr = member.initializer;
             if (!expr) {
-                return !isInAmbientContext(member);
+                return !(member.flags & NodeFlags.Ambient);
             }
             switch (expr.kind) {
                 case SyntaxKind.StringLiteral:
@@ -12738,7 +12738,7 @@ namespace ts {
             const assumeInitialized = isParameter || isAlias || isOuterVariable ||
                 type !== autoType && type !== autoArrayType && (!strictNullChecks || (type.flags & TypeFlags.Any) !== 0 || isInTypeQuery(node) || node.parent.kind === SyntaxKind.ExportSpecifier) ||
                 node.parent.kind === SyntaxKind.NonNullExpression ||
-                isInAmbientContext(declaration);
+                declaration.flags & NodeFlags.Ambient;
             const initialType = assumeInitialized ? (isParameter ? removeOptionalityFromDeclaredType(type, getRootDeclaration(declaration) as VariableLikeDeclaration) : type) :
                 type === autoType || type === autoArrayType ? undefinedType :
                     getNullableType(type, TypeFlags.Undefined);
@@ -15189,7 +15189,7 @@ namespace ts {
             }
             else if (valueDeclaration.kind === SyntaxKind.ClassDeclaration &&
                 node.parent.kind !== SyntaxKind.TypeReference &&
-                !isInAmbientContext(valueDeclaration) &&
+                !(valueDeclaration.flags & NodeFlags.Ambient) &&
                 !isBlockScopedNameDeclaredBeforeUse(valueDeclaration, right)) {
                 error(right, Diagnostics.Class_0_used_before_its_declaration, idText(right));
             }
@@ -17057,7 +17057,7 @@ namespace ts {
             return type;
         }
 
-        function isCommonJsRequire(node: Node) {
+        function isCommonJsRequire(node: Node): boolean {
             if (!isRequireCall(node, /*checkArgumentIsStringLiteral*/ true)) {
                 return false;
             }
@@ -17081,7 +17081,7 @@ namespace ts {
             if (targetDeclarationKind !== SyntaxKind.Unknown) {
                 const decl = getDeclarationOfKind(resolvedRequire, targetDeclarationKind);
                 // function/variable declaration should be ambient
-                return isInAmbientContext(decl);
+                return !!(decl.flags & NodeFlags.Ambient);
             }
             return false;
         }
@@ -19205,7 +19205,7 @@ namespace ts {
                 checkDecorators(node);
                 checkSignatureDeclaration(node);
                 if (node.kind === SyntaxKind.GetAccessor) {
-                    if (!isInAmbientContext(node) && nodeIsPresent(node.body) && (node.flags & NodeFlags.HasImplicitReturn)) {
+                    if (!(node.flags & NodeFlags.Ambient) && nodeIsPresent(node.body) && (node.flags & NodeFlags.HasImplicitReturn)) {
                         if (!(node.flags & NodeFlags.HasExplicitReturn)) {
                             error(node.name, Diagnostics.A_get_accessor_must_return_a_value);
                         }
@@ -19388,7 +19388,7 @@ namespace ts {
         }
 
         function isPrivateWithinAmbient(node: Node): boolean {
-            return hasModifier(node, ModifierFlags.Private) && isInAmbientContext(node);
+            return hasModifier(node, ModifierFlags.Private) && !!(node.flags & NodeFlags.Ambient);
         }
 
         function getEffectiveDeclarationFlags(n: Node, flagsToCheck: ModifierFlags): ModifierFlags {
@@ -19399,7 +19399,7 @@ namespace ts {
             if (n.parent.kind !== SyntaxKind.InterfaceDeclaration &&
                 n.parent.kind !== SyntaxKind.ClassDeclaration &&
                 n.parent.kind !== SyntaxKind.ClassExpression &&
-                isInAmbientContext(n)) {
+                n.flags & NodeFlags.Ambient) {
                 if (!(flags & ModifierFlags.Ambient)) {
                     // It is nested in an ambient context, which means it is automatically exported
                     flags |= ModifierFlags.Export;
@@ -19538,7 +19538,7 @@ namespace ts {
             let multipleConstructorImplementation = false;
             for (const current of declarations) {
                 const node = <FunctionLike>current;
-                const inAmbientContext = isInAmbientContext(node);
+                const inAmbientContext = node.flags & NodeFlags.Ambient;
                 const inAmbientContextOrInterface = node.parent.kind === SyntaxKind.InterfaceDeclaration || node.parent.kind === SyntaxKind.TypeLiteral || inAmbientContext;
                 if (inAmbientContextOrInterface) {
                     // check if declarations are consecutive only if they are non-ambient
@@ -20393,7 +20393,7 @@ namespace ts {
         }
 
         function checkUnusedLocalsAndParameters(node: Node): void {
-            if (node.parent.kind !== SyntaxKind.InterfaceDeclaration && noUnusedIdentifiers && !isInAmbientContext(node)) {
+            if (node.parent.kind !== SyntaxKind.InterfaceDeclaration && noUnusedIdentifiers && !(node.flags & NodeFlags.Ambient)) {
                 node.locals.forEach(local => {
                     if (!local.isReferenced) {
                         if (local.valueDeclaration && getRootDeclaration(local.valueDeclaration).kind === SyntaxKind.Parameter) {
@@ -20446,7 +20446,7 @@ namespace ts {
         }
 
         function checkUnusedClassMembers(node: ClassDeclaration | ClassExpression): void {
-            if (compilerOptions.noUnusedLocals && !isInAmbientContext(node)) {
+            if (compilerOptions.noUnusedLocals && !(node.flags & NodeFlags.Ambient)) {
                 if (node.members) {
                     for (const member of node.members) {
                         if (member.kind === SyntaxKind.MethodDeclaration || member.kind === SyntaxKind.PropertyDeclaration) {
@@ -20467,7 +20467,7 @@ namespace ts {
         }
 
         function checkUnusedTypeParameters(node: ClassDeclaration | ClassExpression | FunctionDeclaration | MethodDeclaration | FunctionExpression | ArrowFunction | ConstructorDeclaration | SignatureDeclaration | InterfaceDeclaration | TypeAliasDeclaration) {
-            if (compilerOptions.noUnusedLocals && !isInAmbientContext(node)) {
+            if (compilerOptions.noUnusedLocals && !(node.flags & NodeFlags.Ambient)) {
                 if (node.typeParameters) {
                     // Only report errors on the last declaration for the type parameter container;
                     // this ensures that all uses have been accounted for.
@@ -20486,7 +20486,7 @@ namespace ts {
         }
 
         function checkUnusedModuleMembers(node: ModuleDeclaration | SourceFile): void {
-            if (compilerOptions.noUnusedLocals && !isInAmbientContext(node)) {
+            if (compilerOptions.noUnusedLocals && !(node.flags & NodeFlags.Ambient)) {
                 node.locals.forEach(local => {
                     if (!local.isReferenced && !local.exportSymbol) {
                         for (const declaration of local.declarations) {
@@ -20519,7 +20519,7 @@ namespace ts {
 
         function checkCollisionWithArgumentsInGeneratedCode(node: SignatureDeclaration) {
             // no rest parameters \ declaration context \ overload - no codegen impact
-            if (!hasRestParameter(node) || isInAmbientContext(node) || nodeIsMissing((<FunctionLikeDeclaration>node).body)) {
+            if (!hasRestParameter(node) || node.flags & NodeFlags.Ambient || nodeIsMissing((<FunctionLikeDeclaration>node).body)) {
                 return;
             }
 
@@ -20545,7 +20545,7 @@ namespace ts {
                 return false;
             }
 
-            if (isInAmbientContext(node)) {
+            if (node.flags & NodeFlags.Ambient) {
                 // ambient context - no codegen impact
                 return false;
             }
@@ -20610,7 +20610,7 @@ namespace ts {
             // bubble up and find containing type
             const enclosingClass = getContainingClass(node);
             // if containing type was not found or it is ambient - exit (no codegen)
-            if (!enclosingClass || isInAmbientContext(enclosingClass)) {
+            if (!enclosingClass || enclosingClass.flags & NodeFlags.Ambient) {
                 return;
             }
 
@@ -21925,7 +21925,7 @@ namespace ts {
             checkClassForDuplicateDeclarations(node);
 
             // Only check for reserved static identifiers on non-ambient context.
-            if (!isInAmbientContext(node)) {
+            if (!(node.flags & NodeFlags.Ambient)) {
                 checkClassForStaticPropertyNameConflicts(node);
             }
 
@@ -22231,7 +22231,7 @@ namespace ts {
             }
             // In ambient enum declarations that specify no const modifier, enum member declarations that omit
             // a value are considered computed members (as opposed to having auto-incremented values).
-            if (isInAmbientContext(member.parent) && !isConst(member.parent)) {
+            if (member.parent.flags & NodeFlags.Ambient && !isConst(member.parent)) {
                 return undefined;
             }
             // If the member declaration specifies no value, the member is considered a constant enum member.
@@ -22264,7 +22264,7 @@ namespace ts {
             else if (isConstEnum) {
                 error(initializer, Diagnostics.In_const_enum_declarations_member_initializer_must_be_constant_expression);
             }
-            else if (isInAmbientContext(member.parent)) {
+            else if (member.parent.flags & NodeFlags.Ambient) {
                 error(initializer, Diagnostics.In_ambient_enum_declarations_member_initializer_must_be_constant_expression);
             }
             else {
@@ -22377,7 +22377,7 @@ namespace ts {
             computeEnumMemberValues(node);
 
             const enumIsConst = isConst(node);
-            if (compilerOptions.isolatedModules && enumIsConst && isInAmbientContext(node)) {
+            if (compilerOptions.isolatedModules && enumIsConst && node.flags & NodeFlags.Ambient) {
                 error(node.name, Diagnostics.Ambient_const_enums_are_not_allowed_when_the_isolatedModules_flag_is_provided);
             }
 
@@ -22429,7 +22429,7 @@ namespace ts {
             for (const declaration of declarations) {
                 if ((declaration.kind === SyntaxKind.ClassDeclaration ||
                     (declaration.kind === SyntaxKind.FunctionDeclaration && nodeIsPresent((<FunctionLikeDeclaration>declaration).body))) &&
-                    !isInAmbientContext(declaration)) {
+                    !(declaration.flags & NodeFlags.Ambient)) {
                     return declaration;
                 }
             }
@@ -22454,7 +22454,7 @@ namespace ts {
             if (produceDiagnostics) {
                 // Grammar checking
                 const isGlobalAugmentation = isGlobalScopeAugmentation(node);
-                const inAmbientContext = isInAmbientContext(node);
+                const inAmbientContext = node.flags & NodeFlags.Ambient;
                 if (isGlobalAugmentation && !inAmbientContext) {
                     error(node.name, Diagnostics.Augmentations_for_the_global_scope_should_have_declare_modifier_unless_they_appear_in_already_ambient_context);
                 }
@@ -22673,7 +22673,7 @@ namespace ts {
                 if (compilerOptions.isolatedModules
                     && node.kind === SyntaxKind.ExportSpecifier
                     && !(target.flags & SymbolFlags.Value)
-                    && !isInAmbientContext(node)) {
+                    && !(node.flags & NodeFlags.Ambient)) {
                     error(node, Diagnostics.Cannot_re_export_a_type_when_the_isolatedModules_flag_is_provided);
                 }
             }
@@ -22724,7 +22724,7 @@ namespace ts {
                 if (hasModifier(node, ModifierFlags.Export)) {
                     markExportAsReferenced(node);
                 }
-                if (isInternalModuleImportEqualsDeclaration(node)) {
+                if (node.moduleReference.kind !== SyntaxKind.ExternalModuleReference) {
                     const target = resolveAlias(getSymbolOfNode(node));
                     if (target !== unknownSymbol) {
                         if (target.flags & SymbolFlags.Value) {
@@ -22740,7 +22740,7 @@ namespace ts {
                     }
                 }
                 else {
-                    if (modulekind >= ModuleKind.ES2015 && !isInAmbientContext(node)) {
+                    if (modulekind >= ModuleKind.ES2015 && !(node.flags & NodeFlags.Ambient)) {
                         // Import equals declaration is deprecated in es6 or above
                         grammarErrorOnNode(node, Diagnostics.Import_assignment_cannot_be_used_when_targeting_ECMAScript_modules_Consider_using_import_Asterisk_as_ns_from_mod_import_a_from_mod_import_d_from_mod_or_another_module_format_instead);
                     }
@@ -22766,7 +22766,7 @@ namespace ts {
 
                     const inAmbientExternalModule = node.parent.kind === SyntaxKind.ModuleBlock && isAmbientModule(node.parent.parent);
                     const inAmbientNamespaceDeclaration = !inAmbientExternalModule && node.parent.kind === SyntaxKind.ModuleBlock &&
-                        !node.moduleSpecifier && isInAmbientContext(node);
+                        !node.moduleSpecifier && node.flags & NodeFlags.Ambient;
                     if (node.parent.kind !== SyntaxKind.SourceFile && !inAmbientExternalModule && !inAmbientNamespaceDeclaration) {
                         error(node, Diagnostics.Export_declarations_are_not_permitted_in_a_namespace);
                     }
@@ -22839,11 +22839,11 @@ namespace ts {
 
             checkExternalModuleExports(container);
 
-            if (isInAmbientContext(node) && !isEntityNameExpression(node.expression)) {
+            if ((node.flags & NodeFlags.Ambient) && !isEntityNameExpression(node.expression)) {
                 grammarErrorOnNode(node.expression, Diagnostics.The_expression_of_an_export_assignment_must_be_an_identifier_or_qualified_name_in_an_ambient_context);
             }
 
-            if (node.isExportEquals && !isInAmbientContext(node)) {
+            if (node.isExportEquals && !(node.flags & NodeFlags.Ambient)) {
                 if (modulekind >= ModuleKind.ES2015) {
                     // export assignment is not supported in es6 modules
                     grammarErrorOnNode(node, Diagnostics.Export_assignment_cannot_be_used_when_targeting_ECMAScript_modules_Consider_using_export_default_or_another_module_format_instead);
@@ -24475,7 +24475,7 @@ namespace ts {
         function checkExternalEmitHelpers(location: Node, helpers: ExternalEmitHelpers) {
             if ((requestedExternalEmitHelpers & helpers) !== helpers && compilerOptions.importHelpers) {
                 const sourceFile = getSourceFileOfNode(location);
-                if (isEffectiveExternalModule(sourceFile, compilerOptions) && !isInAmbientContext(location)) {
+                if (isEffectiveExternalModule(sourceFile, compilerOptions) && !(location.flags & NodeFlags.Ambient)) {
                     const helpersModule = resolveHelpersModule(sourceFile, location);
                     if (helpersModule !== unknownSymbol) {
                         const uncheckedHelpers = helpers & ~requestedExternalEmitHelpers;
@@ -24677,7 +24677,7 @@ namespace ts {
                         else if (node.kind === SyntaxKind.Parameter) {
                             return grammarErrorOnNode(modifier, Diagnostics._0_modifier_cannot_appear_on_a_parameter, "declare");
                         }
-                        else if (isInAmbientContext(node.parent) && node.parent.kind === SyntaxKind.ModuleBlock) {
+                        else if ((node.parent.flags & NodeFlags.Ambient) && node.parent.kind === SyntaxKind.ModuleBlock) {
                             return grammarErrorOnNode(modifier, Diagnostics.A_declare_modifier_cannot_be_used_in_an_already_ambient_context);
                         }
                         flags |= ModifierFlags.Ambient;
@@ -24713,7 +24713,7 @@ namespace ts {
                         if (flags & ModifierFlags.Async) {
                             return grammarErrorOnNode(modifier, Diagnostics._0_modifier_already_seen, "async");
                         }
-                        else if (flags & ModifierFlags.Ambient || isInAmbientContext(node.parent)) {
+                        else if (flags & ModifierFlags.Ambient || node.parent.flags & NodeFlags.Ambient) {
                             return grammarErrorOnNode(modifier, Diagnostics._0_modifier_cannot_be_used_in_an_ambient_context, "async");
                         }
                         else if (node.kind === SyntaxKind.Parameter) {
@@ -25061,7 +25061,7 @@ namespace ts {
                     node.kind === SyntaxKind.FunctionDeclaration ||
                     node.kind === SyntaxKind.FunctionExpression ||
                     node.kind === SyntaxKind.MethodDeclaration);
-                if (isInAmbientContext(node)) {
+                if (node.flags & NodeFlags.Ambient) {
                     return grammarErrorOnNode(node.asteriskToken, Diagnostics.Generators_are_not_allowed_in_an_ambient_context);
                 }
                 if (!node.body) {
@@ -25248,7 +25248,7 @@ namespace ts {
             if (languageVersion < ScriptTarget.ES5) {
                 return grammarErrorOnNode(accessor.name, Diagnostics.Accessors_are_only_available_when_targeting_ECMAScript_5_and_higher);
             }
-            else if (isInAmbientContext(accessor)) {
+            else if (accessor.flags & NodeFlags.Ambient) {
                 return grammarErrorOnNode(accessor.name, Diagnostics.An_accessor_cannot_be_declared_in_an_ambient_context);
             }
             else if (accessor.body === undefined && !hasModifier(accessor, ModifierFlags.Abstract)) {
@@ -25327,7 +25327,7 @@ namespace ts {
                 // However, property declarations disallow computed names in general,
                 // and accessors are not allowed in ambient contexts in general,
                 // so this error only really matters for methods.
-                if (isInAmbientContext(node)) {
+                if (node.flags & NodeFlags.Ambient) {
                     return checkGrammarForNonSymbolComputedProperty(node.name, Diagnostics.A_computed_property_name_in_an_ambient_context_must_directly_refer_to_a_built_in_symbol);
                 }
                 else if (!node.body) {
@@ -25422,7 +25422,7 @@ namespace ts {
 
         function checkGrammarVariableDeclaration(node: VariableDeclaration) {
             if (node.parent.parent.kind !== SyntaxKind.ForInStatement && node.parent.parent.kind !== SyntaxKind.ForOfStatement) {
-                if (isInAmbientContext(node)) {
+                if (node.flags & NodeFlags.Ambient) {
                     if (node.initializer) {
                         if (isConst(node) && !node.type) {
                             if (!isStringOrNumberLiteralExpression(node.initializer)) {
@@ -25452,7 +25452,7 @@ namespace ts {
             }
 
             if (compilerOptions.module !== ModuleKind.ES2015 && compilerOptions.module !== ModuleKind.ESNext && compilerOptions.module !== ModuleKind.System && !compilerOptions.noEmit &&
-                !isInAmbientContext(node.parent.parent) && hasModifier(node.parent.parent, ModifierFlags.Export)) {
+                !(node.parent.parent.flags & NodeFlags.Ambient) && hasModifier(node.parent.parent, ModifierFlags.Export)) {
                 checkESModuleMarker(node.name);
             }
 
@@ -25611,7 +25611,7 @@ namespace ts {
                 }
             }
 
-            if (isInAmbientContext(node) && node.initializer) {
+            if (node.flags & NodeFlags.Ambient && node.initializer) {
                 return grammarErrorOnFirstToken(node.initializer, Diagnostics.Initializers_are_not_allowed_in_ambient_contexts);
             }
         }
@@ -25654,11 +25654,11 @@ namespace ts {
         }
 
         function checkGrammarSourceFile(node: SourceFile): boolean {
-            return isInAmbientContext(node) && checkGrammarTopLevelElementsForRequiredDeclareModifier(node);
+            return !!(node.flags & NodeFlags.Ambient) && checkGrammarTopLevelElementsForRequiredDeclareModifier(node);
         }
 
         function checkGrammarStatementInAmbientContext(node: Node): boolean {
-            if (isInAmbientContext(node)) {
+            if (node.flags & NodeFlags.Ambient) {
                 // An accessors is already reported about the ambient context
                 if (isAccessor(node.parent)) {
                     return getNodeLinks(node).hasReportedStatementInAmbientContext = true;
diff --git a/src/compiler/parser.ts b/src/compiler/parser.ts
index b27d4811ecb43..ebaa89541db5f 100644
--- a/src/compiler/parser.ts
+++ b/src/compiler/parser.ts
@@ -627,6 +627,7 @@ namespace ts {
         }
 
         export function parseIsolatedEntityName(content: string, languageVersion: ScriptTarget): EntityName {
+            // Choice of `isDeclarationFile` should be arbitrary
             initializeState(content, languageVersion, /*syntaxCursor*/ undefined, ScriptKind.JS);
             // Prime the scanner.
             nextToken();
@@ -639,7 +640,7 @@ namespace ts {
         export function parseJsonText(fileName: string, sourceText: string): JsonSourceFile {
             initializeState(sourceText, ScriptTarget.ES2015, /*syntaxCursor*/ undefined, ScriptKind.JSON);
             // Set source file so that errors will be reported with this file name
-            sourceFile = createSourceFile(fileName, ScriptTarget.ES2015, ScriptKind.JSON);
+            sourceFile = createSourceFile(fileName, ScriptTarget.ES2015, ScriptKind.JSON, /*isDeclaration*/ false);
             const result = <JsonSourceFile>sourceFile;
 
             // Prime the scanner.
@@ -681,7 +682,16 @@ namespace ts {
             identifierCount = 0;
             nodeCount = 0;
 
-            contextFlags = scriptKind === ScriptKind.JS || scriptKind === ScriptKind.JSX || scriptKind === ScriptKind.JSON ? NodeFlags.JavaScriptFile : NodeFlags.None;
+            switch (scriptKind) {
+                case ScriptKind.JS:
+                case ScriptKind.JSX:
+                case ScriptKind.JSON:
+                    contextFlags = NodeFlags.JavaScriptFile;
+                    break;
+                default:
+                    contextFlags = NodeFlags.None;
+                    break;
+            }
             parseErrorBeforeNextFinishedNode = false;
 
             // Initialize and prime the scanner before parsing the source elements.
@@ -705,7 +715,12 @@ namespace ts {
         }
 
         function parseSourceFileWorker(fileName: string, languageVersion: ScriptTarget, setParentNodes: boolean, scriptKind: ScriptKind): SourceFile {
-            sourceFile = createSourceFile(fileName, languageVersion, scriptKind);
+            const isDeclarationFile = isDeclarationFileName(fileName);
+            if (isDeclarationFile) {
+                contextFlags |= NodeFlags.Ambient;
+            }
+
+            sourceFile = createSourceFile(fileName, languageVersion, scriptKind, isDeclarationFile);
             sourceFile.flags = contextFlags;
 
             // Prime the scanner.
@@ -782,7 +797,7 @@ namespace ts {
             }
         }
 
-        function createSourceFile(fileName: string, languageVersion: ScriptTarget, scriptKind: ScriptKind): SourceFile {
+        function createSourceFile(fileName: string, languageVersion: ScriptTarget, scriptKind: ScriptKind, isDeclarationFile: boolean): SourceFile {
             // code from createNode is inlined here so createNode won't have to deal with special case of creating source files
             // this is quite rare comparing to other nodes and createNode should be as fast as possible
             const sourceFile = <SourceFile>new SourceFileConstructor(SyntaxKind.SourceFile, /*pos*/ 0, /* end */ sourceText.length);
@@ -793,7 +808,7 @@ namespace ts {
             sourceFile.languageVersion = languageVersion;
             sourceFile.fileName = normalizePath(fileName);
             sourceFile.languageVariant = getLanguageVariant(scriptKind);
-            sourceFile.isDeclarationFile = fileExtensionIs(sourceFile.fileName, Extension.Dts);
+            sourceFile.isDeclarationFile = isDeclarationFile;
             sourceFile.scriptKind = scriptKind;
 
             return sourceFile;
@@ -5089,6 +5104,18 @@ namespace ts {
             const fullStart = getNodePos();
             const decorators = parseDecorators();
             const modifiers = parseModifiers();
+            if (some(modifiers, m => m.kind === SyntaxKind.DeclareKeyword)) {
+                for (const m of modifiers) {
+                    m.flags |= NodeFlags.Ambient;
+                }
+                return doInsideOfContext(NodeFlags.Ambient, () => parseDeclarationWorker(fullStart, decorators, modifiers));
+            }
+            else {
+                return parseDeclarationWorker(fullStart, decorators, modifiers);
+            }
+        }
+
+        function parseDeclarationWorker(fullStart: number, decorators: NodeArray<Decorator> | undefined, modifiers: NodeArray<Modifier> | undefined): Statement {
             switch (token()) {
                 case SyntaxKind.VarKeyword:
                 case SyntaxKind.LetKeyword:
@@ -5446,8 +5473,8 @@ namespace ts {
             return false;
         }
 
-        function parseDecorators(): NodeArray<Decorator> {
-            let list: Decorator[];
+        function parseDecorators(): NodeArray<Decorator> | undefined {
+            let list: Decorator[] | undefined;
             const listPos = getNodePos();
             while (true) {
                 const decoratorStart = getNodePos();
@@ -6131,7 +6158,7 @@ namespace ts {
         export namespace JSDocParser {
             export function parseJSDocTypeExpressionForTests(content: string, start: number, length: number): { jsDocTypeExpression: JSDocTypeExpression, diagnostics: Diagnostic[] } | undefined {
                 initializeState(content, ScriptTarget.Latest, /*_syntaxCursor:*/ undefined, ScriptKind.JS);
-                sourceFile = createSourceFile("file.js", ScriptTarget.Latest, ScriptKind.JS);
+                sourceFile = createSourceFile("file.js", ScriptTarget.Latest, ScriptKind.JS, /*isDeclarationFile*/ false);
                 scanner.setText(content, start, length);
                 currentToken = scanner.scan();
                 const jsDocTypeExpression = parseJSDocTypeExpression();
@@ -7461,4 +7488,8 @@ namespace ts {
             Value = -1
         }
     }
+
+    function isDeclarationFileName(fileName: string): boolean {
+        return fileExtensionIs(fileName, Extension.Dts);
+    }
 }
diff --git a/src/compiler/types.ts b/src/compiler/types.ts
index e1830274c9308..f1387aa40456f 100644
--- a/src/compiler/types.ts
+++ b/src/compiler/types.ts
@@ -451,7 +451,8 @@ namespace ts {
         /* @internal */
         PossiblyContainsDynamicImport = 1 << 19,
         JSDoc =              1 << 20, // If node was parsed inside jsdoc
-        /* @internal */ InWithStatement =    1 << 21, // If any ancestor of node was the `statement` of a WithStatement (not the `expression`)
+        /* @internal */ Ambient =         1 << 21, // If node was inside an ambient context -- a declaration file, or inside something with the `declare` modifier.
+        /* @internal */ InWithStatement = 1 << 22, // If any ancestor of node was the `statement` of a WithStatement (not the `expression`)
 
         BlockScoped = Let | Const,
 
@@ -459,7 +460,7 @@ namespace ts {
         ReachabilityAndEmitFlags = ReachabilityCheckFlags | HasAsyncFunctions,
 
         // Parsing context flags
-        ContextFlags = DisallowInContext | YieldContext | DecoratorContext | AwaitContext | JavaScriptFile | InWithStatement,
+        ContextFlags = DisallowInContext | YieldContext | DecoratorContext | AwaitContext | JavaScriptFile | InWithStatement | Ambient,
 
         // Exclude these flags when parsing a Type
         TypeExcludesFlags = YieldContext | AwaitContext,
diff --git a/src/compiler/utilities.ts b/src/compiler/utilities.ts
index 01bb36e906510..e0c4261f27c83 100644
--- a/src/compiler/utilities.ts
+++ b/src/compiler/utilities.ts
@@ -1718,16 +1718,6 @@ namespace ts {
         return false;
     }
 
-    export function isInAmbientContext(node: Node): boolean {
-        while (node) {
-            if (hasModifier(node, ModifierFlags.Ambient) || (node.kind === SyntaxKind.SourceFile && (node as SourceFile).isDeclarationFile)) {
-                return true;
-            }
-            node = node.parent;
-        }
-        return false;
-    }
-
     // True if the given identifier, string literal, or number literal is the name of a declaration node
     export function isDeclarationName(name: Node): boolean {
         switch (name.kind) {
diff --git a/src/harness/unittests/incrementalParser.ts b/src/harness/unittests/incrementalParser.ts
index fbd8a60da9233..c71b89d3da68d 100644
--- a/src/harness/unittests/incrementalParser.ts
+++ b/src/harness/unittests/incrementalParser.ts
@@ -591,7 +591,7 @@ module m3 { }\
             const index = 0;
             const newTextAndChange = withInsert(oldText, index, "declare ");
 
-            compareTrees(oldText, newTextAndChange.text, newTextAndChange.textChangeRange, 3);
+            compareTrees(oldText, newTextAndChange.text, newTextAndChange.textChangeRange, 0);
         });
 
         it("Insert function above arrow function with comment", () => {
@@ -674,7 +674,7 @@ module m3 { }\
             const oldText = ScriptSnapshot.fromString(source);
             const newTextAndChange = withInsert(oldText, 0, "{");
 
-            compareTrees(oldText, newTextAndChange.text, newTextAndChange.textChangeRange, 9);
+            compareTrees(oldText, newTextAndChange.text, newTextAndChange.textChangeRange, 4);
         });
 
         it("Removing block around function declarations", () => {
@@ -683,7 +683,7 @@ module m3 { }\
             const oldText = ScriptSnapshot.fromString(source);
             const newTextAndChange = withDelete(oldText, 0, "{".length);
 
-            compareTrees(oldText, newTextAndChange.text, newTextAndChange.textChangeRange, 9);
+            compareTrees(oldText, newTextAndChange.text, newTextAndChange.textChangeRange, 4);
         });
 
         it("Moving methods from class to object literal", () => {
diff --git a/src/services/breakpoints.ts b/src/services/breakpoints.ts
index 37aa8df0ca175..3421404a30f35 100644
--- a/src/services/breakpoints.ts
+++ b/src/services/breakpoints.ts
@@ -31,7 +31,7 @@ namespace ts.BreakpointResolver {
         }
 
         // Cannot set breakpoint in ambient declarations
-        if (isInAmbientContext(tokenAtLocation)) {
+        if (tokenAtLocation.flags & NodeFlags.Ambient) {
             return undefined;
         }
 
diff --git a/src/services/refactors/extractSymbol.ts b/src/services/refactors/extractSymbol.ts
index b18e12d2b96da..01b01fa8d4a62 100644
--- a/src/services/refactors/extractSymbol.ts
+++ b/src/services/refactors/extractSymbol.ts
@@ -335,7 +335,7 @@ namespace ts.refactor.extractSymbol {
                 return [createDiagnosticForNode(nodeToCheck, Messages.StatementOrExpressionExpected)];
             }
 
-            if (isInAmbientContext(nodeToCheck)) {
+            if (nodeToCheck.flags & NodeFlags.Ambient) {
                 return [createDiagnosticForNode(nodeToCheck, Messages.CannotExtractAmbientBlock)];
             }
 
diff --git a/src/services/services.ts b/src/services/services.ts
index d6e75868852ae..e307f7deddd64 100644
--- a/src/services/services.ts
+++ b/src/services/services.ts
@@ -41,6 +41,7 @@ namespace ts {
             kind === SyntaxKind.Identifier ? new IdentifierObject(SyntaxKind.Identifier, pos, end) :
                 new TokenObject(kind, pos, end);
         node.parent = parent;
+        node.flags = parent.flags & NodeFlags.ContextFlags;
         return node;
     }
 
diff --git a/src/services/utilities.ts b/src/services/utilities.ts
index 2b116eb1cd05c..d014f48a3f371 100644
--- a/src/services/utilities.ts
+++ b/src/services/utilities.ts
@@ -947,7 +947,7 @@ namespace ts {
         if (flags & ModifierFlags.Static) result.push(ScriptElementKindModifier.staticModifier);
         if (flags & ModifierFlags.Abstract) result.push(ScriptElementKindModifier.abstractModifier);
         if (flags & ModifierFlags.Export) result.push(ScriptElementKindModifier.exportedModifier);
-        if (isInAmbientContext(node)) result.push(ScriptElementKindModifier.ambientModifier);
+        if (node.flags & NodeFlags.Ambient) result.push(ScriptElementKindModifier.ambientModifier);
 
         return result.length > 0 ? result.join(",") : ScriptElementKindModifier.none;
     }
diff --git a/tests/baselines/reference/api/tsserverlibrary.d.ts b/tests/baselines/reference/api/tsserverlibrary.d.ts
index 26c03954b4544..fc5d6790b6484 100644
--- a/tests/baselines/reference/api/tsserverlibrary.d.ts
+++ b/tests/baselines/reference/api/tsserverlibrary.d.ts
@@ -408,7 +408,7 @@ declare namespace ts {
         BlockScoped = 3,
         ReachabilityCheckFlags = 384,
         ReachabilityAndEmitFlags = 1408,
-        ContextFlags = 2193408,
+        ContextFlags = 6387712,
         TypeExcludesFlags = 20480,
     }
     enum ModifierFlags {
diff --git a/tests/baselines/reference/api/typescript.d.ts b/tests/baselines/reference/api/typescript.d.ts
index 6abf9d0cb6e16..94bdeb93339f3 100644
--- a/tests/baselines/reference/api/typescript.d.ts
+++ b/tests/baselines/reference/api/typescript.d.ts
@@ -408,7 +408,7 @@ declare namespace ts {
         BlockScoped = 3,
         ReachabilityCheckFlags = 384,
         ReachabilityAndEmitFlags = 1408,
-        ContextFlags = 2193408,
+        ContextFlags = 6387712,
         TypeExcludesFlags = 20480,
     }
     enum ModifierFlags {
diff --git a/tests/baselines/reference/parserConstructorDeclaration4.errors.txt b/tests/baselines/reference/parserConstructorDeclaration4.errors.txt
index b5329a1cb47ad..bd27ccbb255ee 100644
--- a/tests/baselines/reference/parserConstructorDeclaration4.errors.txt
+++ b/tests/baselines/reference/parserConstructorDeclaration4.errors.txt
@@ -1,12 +1,9 @@
 tests/cases/conformance/parser/ecmascript5/ConstructorDeclarations/parserConstructorDeclaration4.ts(2,3): error TS1031: 'declare' modifier cannot appear on a class element.
-tests/cases/conformance/parser/ecmascript5/ConstructorDeclarations/parserConstructorDeclaration4.ts(2,25): error TS1183: An implementation cannot be declared in ambient contexts.
 
 
-==== tests/cases/conformance/parser/ecmascript5/ConstructorDeclarations/parserConstructorDeclaration4.ts (2 errors) ====
+==== tests/cases/conformance/parser/ecmascript5/ConstructorDeclarations/parserConstructorDeclaration4.ts (1 errors) ====
     class C {
       declare constructor() { }
       ~~~~~~~
 !!! error TS1031: 'declare' modifier cannot appear on a class element.
-                            ~
-!!! error TS1183: An implementation cannot be declared in ambient contexts.
     }
\ No newline at end of file
diff --git a/tests/baselines/reference/parserMemberAccessorDeclaration11.errors.txt b/tests/baselines/reference/parserMemberAccessorDeclaration11.errors.txt
index 5da5e536e067a..3a04bfbbd0296 100644
--- a/tests/baselines/reference/parserMemberAccessorDeclaration11.errors.txt
+++ b/tests/baselines/reference/parserMemberAccessorDeclaration11.errors.txt
@@ -1,9 +1,12 @@
 tests/cases/conformance/parser/ecmascript5/MemberAccessorDeclarations/parserMemberAccessorDeclaration11.ts(2,5): error TS1031: 'declare' modifier cannot appear on a class element.
+tests/cases/conformance/parser/ecmascript5/MemberAccessorDeclarations/parserMemberAccessorDeclaration11.ts(2,17): error TS2378: A 'get' accessor must return a value.
 
 
-==== tests/cases/conformance/parser/ecmascript5/MemberAccessorDeclarations/parserMemberAccessorDeclaration11.ts (1 errors) ====
+==== tests/cases/conformance/parser/ecmascript5/MemberAccessorDeclarations/parserMemberAccessorDeclaration11.ts (2 errors) ====
     class C {
         declare get Foo() { }
         ~~~~~~~
 !!! error TS1031: 'declare' modifier cannot appear on a class element.
+                    ~~~
+!!! error TS2378: A 'get' accessor must return a value.
     }
\ No newline at end of file
diff --git a/tests/baselines/reference/parserMemberFunctionDeclaration5.errors.txt b/tests/baselines/reference/parserMemberFunctionDeclaration5.errors.txt
index 402355919a031..dc19cb0f8550a 100644
--- a/tests/baselines/reference/parserMemberFunctionDeclaration5.errors.txt
+++ b/tests/baselines/reference/parserMemberFunctionDeclaration5.errors.txt
@@ -1,12 +1,9 @@
 tests/cases/conformance/parser/ecmascript5/MemberFunctionDeclarations/parserMemberFunctionDeclaration5.ts(2,5): error TS1031: 'declare' modifier cannot appear on a class element.
-tests/cases/conformance/parser/ecmascript5/MemberFunctionDeclarations/parserMemberFunctionDeclaration5.ts(2,19): error TS1183: An implementation cannot be declared in ambient contexts.
 
 
-==== tests/cases/conformance/parser/ecmascript5/MemberFunctionDeclarations/parserMemberFunctionDeclaration5.ts (2 errors) ====
+==== tests/cases/conformance/parser/ecmascript5/MemberFunctionDeclarations/parserMemberFunctionDeclaration5.ts (1 errors) ====
     class C {
         declare Foo() { }
         ~~~~~~~
 !!! error TS1031: 'declare' modifier cannot appear on a class element.
-                      ~
-!!! error TS1183: An implementation cannot be declared in ambient contexts.
     }
\ No newline at end of file