diff --git a/src/compiler/commandLineParser.ts b/src/compiler/commandLineParser.ts
index 968e5bb756eaa..782f76b3d1fa5 100644
--- a/src/compiler/commandLineParser.ts
+++ b/src/compiler/commandLineParser.ts
@@ -1146,7 +1146,7 @@ namespace ts {
     }
 
     /* @internal */
-    export function printHelp(optionsList: CommandLineOption[], syntaxPrefix = "") {
+    export function printHelp(optionsList: ReadonlyArray<CommandLineOption>, syntaxPrefix = "") {
         const output: string[] = [];
 
         // We want to align our "syntax" and "examples" commands to a certain margin.
diff --git a/src/compiler/core.ts b/src/compiler/core.ts
index 6288f30b14472..8c976cee9ee56 100644
--- a/src/compiler/core.ts
+++ b/src/compiler/core.ts
@@ -16,6 +16,10 @@ namespace ts {
         [index: string]: T;
     }
 
+    export interface SortedReadonlyArray<T> extends ReadonlyArray<T> {
+        " __sortedArrayBrand": any;
+    }
+
     export interface SortedArray<T> extends Array<T> {
         " __sortedArrayBrand": any;
     }
@@ -815,8 +819,8 @@ namespace ts {
     /**
      * Deduplicates an array that has already been sorted.
      */
-    function deduplicateSorted<T>(array: ReadonlyArray<T>, comparer: EqualityComparer<T> | Comparer<T>): T[] {
-        if (array.length === 0) return [];
+    function deduplicateSorted<T>(array: SortedReadonlyArray<T>, comparer: EqualityComparer<T> | Comparer<T>): SortedReadonlyArray<T> {
+        if (array.length === 0) return emptyArray as any as SortedReadonlyArray<T>;
 
         let last = array[0];
         const deduplicated: T[] = [last];
@@ -838,7 +842,7 @@ namespace ts {
             deduplicated.push(last = next);
         }
 
-        return deduplicated;
+        return deduplicated as any as SortedReadonlyArray<T>;
     }
 
     export function insertSorted<T>(array: SortedArray<T>, insert: T, compare: Comparer<T>): void {
@@ -853,8 +857,10 @@ namespace ts {
         }
     }
 
-    export function sortAndDeduplicate<T>(array: ReadonlyArray<T>, comparer: Comparer<T>, equalityComparer?: EqualityComparer<T>) {
-        return deduplicateSorted(sort(array, comparer), equalityComparer || comparer);
+    export function sortAndDeduplicate<T>(array: ReadonlyArray<string>): SortedReadonlyArray<string>;
+    export function sortAndDeduplicate<T>(array: ReadonlyArray<T>, comparer: Comparer<T>, equalityComparer?: EqualityComparer<T>): SortedReadonlyArray<T>;
+    export function sortAndDeduplicate<T>(array: ReadonlyArray<T>, comparer?: Comparer<T>, equalityComparer?: EqualityComparer<T>): SortedReadonlyArray<T> {
+        return deduplicateSorted(sort(array, comparer), equalityComparer || comparer || compareStringsCaseSensitive as any as Comparer<T>);
     }
 
     export function arrayIsEqualTo<T>(array1: ReadonlyArray<T> | undefined, array2: ReadonlyArray<T> | undefined, equalityComparer: (a: T, b: T, index: number) => boolean = equateValues): boolean {
@@ -1035,8 +1041,8 @@ namespace ts {
     /**
      * Returns a new sorted array.
      */
-    export function sort<T>(array: ReadonlyArray<T>, comparer: Comparer<T>): T[] {
-        return array.slice().sort(comparer);
+    export function sort<T>(array: ReadonlyArray<T>, comparer?: Comparer<T>): SortedReadonlyArray<T> {
+        return (array.length === 0 ? array : array.slice().sort(comparer)) as SortedReadonlyArray<T>;
     }
 
     export function arrayIterator<T>(array: ReadonlyArray<T>): Iterator<T> {
diff --git a/src/compiler/program.ts b/src/compiler/program.ts
index 85c6b27f4ec7d..997bdaa80c726 100644
--- a/src/compiler/program.ts
+++ b/src/compiler/program.ts
@@ -198,7 +198,7 @@ namespace ts {
         };
     }
 
-    export function getPreEmitDiagnostics(program: Program, sourceFile?: SourceFile, cancellationToken?: CancellationToken): Diagnostic[] {
+    export function getPreEmitDiagnostics(program: Program, sourceFile?: SourceFile, cancellationToken?: CancellationToken): ReadonlyArray<Diagnostic> {
         const diagnostics = [
             ...program.getConfigFileParsingDiagnostics(),
             ...program.getOptionsDiagnostics(cancellationToken),
@@ -1817,7 +1817,7 @@ namespace ts {
             return sourceFile.isDeclarationFile ? [] : getDeclarationDiagnosticsWorker(sourceFile, cancellationToken);
         }
 
-        function getOptionsDiagnostics(): Diagnostic[] {
+        function getOptionsDiagnostics(): SortedReadonlyArray<Diagnostic> {
             return sortAndDeduplicateDiagnostics(concatenate(
                 fileProcessingDiagnostics.getGlobalDiagnostics(),
                 concatenate(
@@ -1838,7 +1838,7 @@ namespace ts {
             return diagnostics;
         }
 
-        function getGlobalDiagnostics(): Diagnostic[] {
+        function getGlobalDiagnostics(): SortedReadonlyArray<Diagnostic> {
             return sortAndDeduplicateDiagnostics(getDiagnosticsProducingTypeChecker().getGlobalDiagnostics().slice());
         }
 
diff --git a/src/compiler/utilities.ts b/src/compiler/utilities.ts
index 96b9c3c2f67ef..a969d580b86a9 100644
--- a/src/compiler/utilities.ts
+++ b/src/compiler/utilities.ts
@@ -7,7 +7,7 @@ namespace ts {
         return pathIsRelative(moduleName) || isRootedDiskPath(moduleName);
     }
 
-    export function sortAndDeduplicateDiagnostics<T extends Diagnostic>(diagnostics: ReadonlyArray<T>): T[] {
+    export function sortAndDeduplicateDiagnostics<T extends Diagnostic>(diagnostics: ReadonlyArray<T>): SortedReadonlyArray<T> {
         return sortAndDeduplicate<T>(diagnostics, compareDiagnostics);
     }
 }
diff --git a/src/harness/compiler.ts b/src/harness/compiler.ts
index 7d41545708ba7..532a2d655789a 100644
--- a/src/harness/compiler.ts
+++ b/src/harness/compiler.ts
@@ -58,7 +58,7 @@ namespace compiler {
         private _inputs: documents.TextDocument[] = [];
         private _inputsAndOutputs: collections.SortedMap<string, CompilationOutput>;
 
-        constructor(host: fakes.CompilerHost, options: ts.CompilerOptions, program: ts.Program | undefined, result: ts.EmitResult | undefined, diagnostics: ts.Diagnostic[]) {
+        constructor(host: fakes.CompilerHost, options: ts.CompilerOptions, program: ts.Program | undefined, result: ts.EmitResult | undefined, diagnostics: ReadonlyArray<ts.Diagnostic>) {
             this.host = host;
             this.program = program;
             this.result = result;
diff --git a/src/jsTyping/types.ts b/src/jsTyping/types.ts
index 408d30a65118a..6bc3e667f8766 100644
--- a/src/jsTyping/types.ts
+++ b/src/jsTyping/types.ts
@@ -8,10 +8,6 @@ declare namespace ts.server {
     export type EventEndInstallTypes = "event::endInstallTypes";
     export type EventInitializationFailed = "event::initializationFailed";
 
-    export interface SortedReadonlyArray<T> extends ReadonlyArray<T> {
-        " __sortedArrayBrand": any;
-    }
-
     export interface TypingInstallerResponse {
         readonly kind: ActionSet | ActionInvalidate | EventTypesRegistry | ActionPackageInstalled | ActionValueInspected | EventBeginInstallTypes | EventEndInstallTypes | EventInitializationFailed;
     }
diff --git a/src/server/project.ts b/src/server/project.ts
index fb6ec4b80fc74..ff2e3539b6bbc 100644
--- a/src/server/project.ts
+++ b/src/server/project.ts
@@ -554,7 +554,7 @@ namespace ts.server {
         }
 
         getExternalFiles(): SortedReadonlyArray<string> {
-            return toSortedArray(flatMapToMutable(this.plugins, plugin => {
+            return sort(flatMap(this.plugins, plugin => {
                 if (typeof plugin.module.getExternalFiles !== "function") return;
                 try {
                     return plugin.module.getExternalFiles(this);
@@ -871,7 +871,7 @@ namespace ts.server {
                             (result || (result = [])).push(...unResolved);
                         }
                     }
-                    this.lastCachedUnresolvedImportsList = result ? toDeduplicatedSortedArray(result) : emptyArray;
+                    this.lastCachedUnresolvedImportsList = result ? sortAndDeduplicate(result) : emptyArray;
                 }
 
                 this.projectService.typingsCache.enqueueInstallTypingsForProject(this, this.lastCachedUnresolvedImportsList, hasAddedorRemovedFiles);
@@ -888,7 +888,7 @@ namespace ts.server {
 
         /*@internal*/
         updateTypingFiles(typingFiles: SortedReadonlyArray<string>) {
-            enumerateInsertsAndDeletes(typingFiles, this.typingFiles, getStringComparer(!this.useCaseSensitiveFileNames()),
+            enumerateInsertsAndDeletes<string, string>(typingFiles, this.typingFiles, getStringComparer(!this.useCaseSensitiveFileNames()),
                 /*inserted*/ noop,
                 removed => this.detachScriptInfoFromProject(removed)
             );
@@ -959,7 +959,7 @@ namespace ts.server {
 
             const oldExternalFiles = this.externalFiles || emptyArray as SortedReadonlyArray<string>;
             this.externalFiles = this.getExternalFiles();
-            enumerateInsertsAndDeletes(this.externalFiles, oldExternalFiles, getStringComparer(!this.useCaseSensitiveFileNames()),
+            enumerateInsertsAndDeletes<string, string>(this.externalFiles, oldExternalFiles, getStringComparer(!this.useCaseSensitiveFileNames()),
                 // Ensure a ScriptInfo is created for new external files. This is performed indirectly
                 // by the LSHost for files in the program when the program is retrieved above but
                 // the program doesn't contain external files so this must be done explicitly.
diff --git a/src/server/typingsCache.ts b/src/server/typingsCache.ts
index 64a117bbe258e..d3b6e21a21c94 100644
--- a/src/server/typingsCache.ts
+++ b/src/server/typingsCache.ts
@@ -130,7 +130,7 @@ namespace ts.server {
         }
 
         updateTypingsForProject(projectName: string, compilerOptions: CompilerOptions, typeAcquisition: TypeAcquisition, unresolvedImports: SortedReadonlyArray<string>, newTypings: string[]) {
-            const typings = toSortedArray(newTypings);
+            const typings = sort(newTypings);
             this.perProjectCache.set(projectName, {
                 compilerOptions,
                 typeAcquisition,
diff --git a/src/server/utilities.ts b/src/server/utilities.ts
index 6dcfff6f7e22a..4eac1730b7704 100644
--- a/src/server/utilities.ts
+++ b/src/server/utilities.ts
@@ -206,22 +206,6 @@ namespace ts.server {
         }
     }
 
-    export function toSortedArray(arr: string[]): SortedArray<string>;
-    export function toSortedArray<T>(arr: T[], comparer: Comparer<T>): SortedArray<T>;
-    export function toSortedArray<T>(arr: T[], comparer?: Comparer<T>): SortedArray<T> {
-        arr.sort(comparer);
-        return arr as SortedArray<T>;
-    }
-
-    export function toDeduplicatedSortedArray(arr: string[]): SortedArray<string> {
-        arr.sort();
-        filterMutate(arr, isNonDuplicateInSortedArray);
-        return arr as SortedArray<string>;
-    }
-    function isNonDuplicateInSortedArray<T>(value: T, index: number, array: T[]) {
-        return index === 0 || value !== array[index - 1];
-    }
-
     const indentStr = "\n    ";
 
     /* @internal */
diff --git a/src/testRunner/projectsRunner.ts b/src/testRunner/projectsRunner.ts
index 3dcf1911dde80..2d15133f399af 100644
--- a/src/testRunner/projectsRunner.ts
+++ b/src/testRunner/projectsRunner.ts
@@ -310,9 +310,7 @@ namespace project {
             const program = ts.createProgram(getInputFiles(), compilerOptions, compilerHost);
             const errors = ts.getPreEmitDiagnostics(program);
 
-            const emitResult = program.emit();
-            ts.addRange(errors, emitResult.diagnostics);
-            const sourceMapData = emitResult.sourceMaps;
+            const { sourceMaps: sourceMapData, diagnostics: emitDiagnostics } = program.emit();
 
             // Clean up source map data that will be used in baselining
             if (sourceMapData) {
@@ -329,7 +327,7 @@ namespace project {
                 configFileSourceFiles,
                 moduleKind,
                 program,
-                errors,
+                errors: ts.concatenate(errors, emitDiagnostics),
                 sourceMapData
             };
         }
diff --git a/src/testRunner/unittests/tsserverProjectSystem.ts b/src/testRunner/unittests/tsserverProjectSystem.ts
index 38d2d4867a0ed..3ffd28702a2db 100644
--- a/src/testRunner/unittests/tsserverProjectSystem.ts
+++ b/src/testRunner/unittests/tsserverProjectSystem.ts
@@ -118,7 +118,7 @@ namespace ts.projectSystem {
             this.projectService.updateTypingsForProject(response);
         }
 
-        enqueueInstallTypingsRequest(project: server.Project, typeAcquisition: TypeAcquisition, unresolvedImports: server.SortedReadonlyArray<string>) {
+        enqueueInstallTypingsRequest(project: server.Project, typeAcquisition: TypeAcquisition, unresolvedImports: SortedReadonlyArray<string>) {
             const request = server.createInstallTypingsRequest(project, typeAcquisition, unresolvedImports, this.globalTypingsCacheLocation);
             this.install(request);
         }
diff --git a/src/testRunner/unittests/typingsInstaller.ts b/src/testRunner/unittests/typingsInstaller.ts
index a718a12f5484f..b06c22d5d7912 100644
--- a/src/testRunner/unittests/typingsInstaller.ts
+++ b/src/testRunner/unittests/typingsInstaller.ts
@@ -310,7 +310,7 @@ namespace ts.projectSystem {
                 constructor() {
                     super(host, { typesRegistry: createTypesRegistry("jquery") });
                 }
-                enqueueInstallTypingsRequest(project: server.Project, typeAcquisition: TypeAcquisition, unresolvedImports: server.SortedReadonlyArray<string>) {
+                enqueueInstallTypingsRequest(project: server.Project, typeAcquisition: TypeAcquisition, unresolvedImports: SortedReadonlyArray<string>) {
                     enqueueIsCalled = true;
                     super.enqueueInstallTypingsRequest(project, typeAcquisition, unresolvedImports);
                 }
@@ -409,7 +409,7 @@ namespace ts.projectSystem {
                 constructor() {
                     super(host, { typesRegistry: createTypesRegistry("jquery") });
                 }
-                enqueueInstallTypingsRequest(project: server.Project, typeAcquisition: TypeAcquisition, unresolvedImports: server.SortedReadonlyArray<string>) {
+                enqueueInstallTypingsRequest(project: server.Project, typeAcquisition: TypeAcquisition, unresolvedImports: SortedReadonlyArray<string>) {
                     super.enqueueInstallTypingsRequest(project, typeAcquisition, unresolvedImports);
                 }
                 installWorker(_requestId: number, _args: string[], _cwd: string, cb: TI.RequestCompletedAction): void {
@@ -451,7 +451,7 @@ namespace ts.projectSystem {
                 constructor() {
                     super(host, { typesRegistry: createTypesRegistry("jquery") });
                 }
-                enqueueInstallTypingsRequest(project: server.Project, typeAcquisition: TypeAcquisition, unresolvedImports: server.SortedReadonlyArray<string>) {
+                enqueueInstallTypingsRequest(project: server.Project, typeAcquisition: TypeAcquisition, unresolvedImports: SortedReadonlyArray<string>) {
                     super.enqueueInstallTypingsRequest(project, typeAcquisition, unresolvedImports);
                 }
                 installWorker(_requestId: number, _args: string[], _cwd: string, cb: TI.RequestCompletedAction): void {
diff --git a/tests/baselines/reference/api/tsserverlibrary.d.ts b/tests/baselines/reference/api/tsserverlibrary.d.ts
index ae04e45229a85..2ea0d6777095b 100644
--- a/tests/baselines/reference/api/tsserverlibrary.d.ts
+++ b/tests/baselines/reference/api/tsserverlibrary.d.ts
@@ -27,6 +27,9 @@ declare namespace ts {
     interface MapLike<T> {
         [index: string]: T;
     }
+    interface SortedReadonlyArray<T> extends ReadonlyArray<T> {
+        " __sortedArrayBrand": any;
+    }
     interface SortedArray<T> extends Array<T> {
         " __sortedArrayBrand": any;
     }
@@ -3126,7 +3129,7 @@ declare namespace ts {
 /** Non-internal stuff goes here */
 declare namespace ts {
     function isExternalModuleNameRelative(moduleName: string): boolean;
-    function sortAndDeduplicateDiagnostics<T extends Diagnostic>(diagnostics: ReadonlyArray<T>): T[];
+    function sortAndDeduplicateDiagnostics<T extends Diagnostic>(diagnostics: ReadonlyArray<T>): SortedReadonlyArray<T>;
 }
 declare namespace ts {
     function getDefaultLibFileName(options: CompilerOptions): string;
@@ -4153,7 +4156,7 @@ declare namespace ts {
     function findConfigFile(searchPath: string, fileExists: (fileName: string) => boolean, configName?: string): string | undefined;
     function resolveTripleslashReference(moduleName: string, containingFile: string): string;
     function createCompilerHost(options: CompilerOptions, setParentNodes?: boolean): CompilerHost;
-    function getPreEmitDiagnostics(program: Program, sourceFile?: SourceFile, cancellationToken?: CancellationToken): Diagnostic[];
+    function getPreEmitDiagnostics(program: Program, sourceFile?: SourceFile, cancellationToken?: CancellationToken): ReadonlyArray<Diagnostic>;
     interface FormatDiagnosticsHost {
         getCurrentDirectory(): string;
         getCanonicalFileName(fileName: string): string;
@@ -4457,9 +4460,6 @@ declare namespace ts.server {
     type EventBeginInstallTypes = "event::beginInstallTypes";
     type EventEndInstallTypes = "event::endInstallTypes";
     type EventInitializationFailed = "event::initializationFailed";
-    interface SortedReadonlyArray<T> extends ReadonlyArray<T> {
-        " __sortedArrayBrand": any;
-    }
     interface TypingInstallerResponse {
         readonly kind: ActionSet | ActionInvalidate | EventTypesRegistry | ActionPackageInstalled | ActionValueInspected | EventBeginInstallTypes | EventEndInstallTypes | EventInitializationFailed;
     }
diff --git a/tests/baselines/reference/api/typescript.d.ts b/tests/baselines/reference/api/typescript.d.ts
index fd4f575261400..91c8c3975017f 100644
--- a/tests/baselines/reference/api/typescript.d.ts
+++ b/tests/baselines/reference/api/typescript.d.ts
@@ -27,6 +27,9 @@ declare namespace ts {
     interface MapLike<T> {
         [index: string]: T;
     }
+    interface SortedReadonlyArray<T> extends ReadonlyArray<T> {
+        " __sortedArrayBrand": any;
+    }
     interface SortedArray<T> extends Array<T> {
         " __sortedArrayBrand": any;
     }
@@ -3126,7 +3129,7 @@ declare namespace ts {
 /** Non-internal stuff goes here */
 declare namespace ts {
     function isExternalModuleNameRelative(moduleName: string): boolean;
-    function sortAndDeduplicateDiagnostics<T extends Diagnostic>(diagnostics: ReadonlyArray<T>): T[];
+    function sortAndDeduplicateDiagnostics<T extends Diagnostic>(diagnostics: ReadonlyArray<T>): SortedReadonlyArray<T>;
 }
 declare namespace ts {
     function getDefaultLibFileName(options: CompilerOptions): string;
@@ -4153,7 +4156,7 @@ declare namespace ts {
     function findConfigFile(searchPath: string, fileExists: (fileName: string) => boolean, configName?: string): string | undefined;
     function resolveTripleslashReference(moduleName: string, containingFile: string): string;
     function createCompilerHost(options: CompilerOptions, setParentNodes?: boolean): CompilerHost;
-    function getPreEmitDiagnostics(program: Program, sourceFile?: SourceFile, cancellationToken?: CancellationToken): Diagnostic[];
+    function getPreEmitDiagnostics(program: Program, sourceFile?: SourceFile, cancellationToken?: CancellationToken): ReadonlyArray<Diagnostic>;
     interface FormatDiagnosticsHost {
         getCurrentDirectory(): string;
         getCanonicalFileName(fileName: string): string;
@@ -4457,9 +4460,6 @@ declare namespace ts.server {
     type EventBeginInstallTypes = "event::beginInstallTypes";
     type EventEndInstallTypes = "event::endInstallTypes";
     type EventInitializationFailed = "event::initializationFailed";
-    interface SortedReadonlyArray<T> extends ReadonlyArray<T> {
-        " __sortedArrayBrand": any;
-    }
     interface TypingInstallerResponse {
         readonly kind: ActionSet | ActionInvalidate | EventTypesRegistry | ActionPackageInstalled | ActionValueInspected | EventBeginInstallTypes | EventEndInstallTypes | EventInitializationFailed;
     }