Works without experimental flow

src/error/GraphQLError.js

@@ -124,8 +124,9 @@ export function GraphQLError( // eslint-disable-line no-redeclare
   }
 
   let _locations;
-  if (positions && source) {
-    _locations = positions.map(pos => getLocation(source, pos));
+  const immutableSource = source;
+  if (positions && immutableSource) {
+    _locations = positions.map(pos => getLocation(immutableSource, pos));
   } else if (_nodes) {
     _locations = _nodes.reduce((list, node) => {
       if (node.loc) {

src/type/introspection.js

@@ -22,7 +22,6 @@ import {
   isListType,
   isNonNullType,
   isAbstractType,
-  isNamedType,
 } from './definition';
 import { GraphQLList, GraphQLNonNull } from '../type/wrappers';
 import { GraphQLString, GraphQLBoolean } from './scalars';
@@ -467,9 +466,23 @@ export const introspectionTypes: $ReadOnlyArray<*> = [
 
 export function isIntrospectionType(type: ?GraphQLType): boolean %checks {
   return (
-    isNamedType(type) &&
-    introspectionTypes.some(
-      introspectionType => introspectionType.name === type.name,
-    )
+    // Cannot use function calls to %checks unless flow's
+    // experimental.const_params=true is set.
+
+    // isNamedType(type)
+    type !== null &&
+    type !== undefined &&
+    !(type instanceof GraphQLList || type instanceof GraphQLNonNull) &&
+    // introspectionTypes.some(
+    //   introspectionType => introspectionType.name === type.name
+    // )
+    (__Schema.name === type.name ||
+      __Directive.name === type.name ||
+      __DirectiveLocation.name === type.name ||
+      __Type.name === type.name ||
+      __Field.name === type.name ||
+      __InputValue.name === type.name ||
+      __EnumValue.name === type.name ||
+      __TypeKind.name === type.name)
   );
 }

src/type/scalars.js

@@ -7,7 +7,7 @@
  * @flow
  */
 
-import { GraphQLScalarType, isNamedType } from './definition';
+import { GraphQLScalarType } from './definition';
 import * as Kind from '../language/kinds';
 import type { GraphQLType } from './definition';
 
@@ -145,11 +145,22 @@ export const specifiedScalarTypes: $ReadOnlyArray<*> = [
   GraphQLID,
 ];
 
-export function isSpecifiedScalarType(type: ?GraphQLType): boolean %checks {
+export function isSpecifiedScalarType(
+  type: $ReadOnly<?GraphQLType>,
+): boolean %checks {
   return (
-    isNamedType(type) &&
-    specifiedScalarTypes.some(
-      specifiedScalarType => specifiedScalarType.name === type.name,
-    )
+    // Cannot use function calls to %checks unless flow's
+    // experimental.const_params=true is set.
+
+    // equivalent of isNamedType(type), given we only care about scalars
+    type instanceof GraphQLScalarType &&
+    // specifiedScalarTypes.some(
+    //   specifiedScalarType => specifiedScalarType.name === type.name
+    // )
+    (GraphQLString.name === type.name ||
+      GraphQLInt.name === type.name ||
+      GraphQLFloat.name === type.name ||
+      GraphQLBoolean.name === type.name ||
+      GraphQLID.name === type.name)
   );
 }