Motivation: looked into this it as part of #3687 (#3690)

Explanation: `Object.create(null)` returns value of `any` type.
So bellow construct is not reported by TS even in "strict" mode:
```ts
const foo = Object.create(null);
```
Fixing this issue in `extendSchema` requires adding more code since we
can't put all extensions nodes into one collection without loosing
typesafety.

src/utilities/extendSchema.ts

@@ -1,3 +1,4 @@
+import { AccumulatorMap } from '../jsutils/AccumulatorMap';
 import { inspect } from '../jsutils/inspect';
 import { invariant } from '../jsutils/invariant';
 import { keyMap } from '../jsutils/keyMap';
@@ -29,10 +30,6 @@ import type {
   UnionTypeExtensionNode,
 } from '../language/ast';
 import { Kind } from '../language/kinds';
-import {
-  isTypeDefinitionNode,
-  isTypeExtensionNode,
-} from '../language/predicates';
 
 import type {
   GraphQLArgumentConfig,
@@ -131,7 +128,25 @@ export function extendSchemaImpl(
 ): GraphQLSchemaNormalizedConfig {
   // Collect the type definitions and extensions found in the document.
   const typeDefs: Array<TypeDefinitionNode> = [];
-  const typeExtensionsMap = Object.create(null);
+
+  const scalarExtensions = new AccumulatorMap<
+    string,
+    ScalarTypeExtensionNode
+  >();
+  const objectExtensions = new AccumulatorMap<
+    string,
+    ObjectTypeExtensionNode
+  >();
+  const interfaceExtensions = new AccumulatorMap<
+    string,
+    InterfaceTypeExtensionNode
+  >();
+  const unionExtensions = new AccumulatorMap<string, UnionTypeExtensionNode>();
+  const enumExtensions = new AccumulatorMap<string, EnumTypeExtensionNode>();
+  const inputObjectExtensions = new AccumulatorMap<
+    string,
+    InputObjectTypeExtensionNode
+  >();
 
   // New directives and types are separate because a directives and types can
   // have the same name. For example, a type named "skip".
@@ -141,33 +156,57 @@ export function extendSchemaImpl(
   // Schema extensions are collected which may add additional operation types.
   const schemaExtensions: Array<SchemaExtensionNode> = [];
 
+  let isSchemaChanged = false;
   for (const def of documentAST.definitions) {
-    if (def.kind === Kind.SCHEMA_DEFINITION) {
-      schemaDef = def;
-    } else if (def.kind === Kind.SCHEMA_EXTENSION) {
-      schemaExtensions.push(def);
-    } else if (isTypeDefinitionNode(def)) {
-      typeDefs.push(def);
-    } else if (isTypeExtensionNode(def)) {
-      const extendedTypeName = def.name.value;
-      const existingTypeExtensions = typeExtensionsMap[extendedTypeName];
-      typeExtensionsMap[extendedTypeName] = existingTypeExtensions
-        ? existingTypeExtensions.concat([def])
-        : [def];
-    } else if (def.kind === Kind.DIRECTIVE_DEFINITION) {
-      directiveDefs.push(def);
+    switch (def.kind) {
+      case Kind.SCHEMA_DEFINITION:
+        schemaDef = def;
+        break;
+      case Kind.SCHEMA_EXTENSION:
+        schemaExtensions.push(def);
+        break;
+      case Kind.DIRECTIVE_DEFINITION:
+        directiveDefs.push(def);
+        break;
+
+      // Type Definitions
+      case Kind.SCALAR_TYPE_DEFINITION:
+      case Kind.OBJECT_TYPE_DEFINITION:
+      case Kind.INTERFACE_TYPE_DEFINITION:
+      case Kind.UNION_TYPE_DEFINITION:
+      case Kind.ENUM_TYPE_DEFINITION:
+      case Kind.INPUT_OBJECT_TYPE_DEFINITION:
+        typeDefs.push(def);
+        break;
+
+      // Type System Extensions
+      case Kind.SCALAR_TYPE_EXTENSION:
+        scalarExtensions.add(def.name.value, def);
+        break;
+      case Kind.OBJECT_TYPE_EXTENSION:
+        objectExtensions.add(def.name.value, def);
+        break;
+      case Kind.INTERFACE_TYPE_EXTENSION:
+        interfaceExtensions.add(def.name.value, def);
+        break;
+      case Kind.UNION_TYPE_EXTENSION:
+        unionExtensions.add(def.name.value, def);
+        break;
+      case Kind.ENUM_TYPE_EXTENSION:
+        enumExtensions.add(def.name.value, def);
+        break;
+      case Kind.INPUT_OBJECT_TYPE_EXTENSION:
+        inputObjectExtensions.add(def.name.value, def);
+        break;
+      default:
+        continue;
     }
+    isSchemaChanged = true;
   }
 
   // If this document contains no new types, extensions, or directives then
   // return the same unmodified GraphQLSchema instance.
-  if (
-    Object.keys(typeExtensionsMap).length === 0 &&
-    typeDefs.length === 0 &&
-    directiveDefs.length === 0 &&
-    schemaExtensions.length === 0 &&
-    schemaDef == null
-  ) {
+  if (!isSchemaChanged) {
     return schemaConfig;
   }
 
@@ -275,7 +314,7 @@ export function extendSchemaImpl(
     type: GraphQLInputObjectType,
   ): GraphQLInputObjectType {
     const config = type.toConfig();
-    const extensions = typeExtensionsMap[config.name] ?? [];
+    const extensions = inputObjectExtensions.get(config.name) ?? [];
 
     return new GraphQLInputObjectType({
       ...config,
@@ -292,7 +331,7 @@ export function extendSchemaImpl(
 
   function extendEnumType(type: GraphQLEnumType): GraphQLEnumType {
     const config = type.toConfig();
-    const extensions = typeExtensionsMap[type.name] ?? [];
+    const extensions = enumExtensions.get(type.name) ?? [];
 
     return new GraphQLEnumType({
       ...config,
@@ -306,7 +345,7 @@ export function extendSchemaImpl(
 
   function extendScalarType(type: GraphQLScalarType): GraphQLScalarType {
     const config = type.toConfig();
-    const extensions = typeExtensionsMap[config.name] ?? [];
+    const extensions = scalarExtensions.get(config.name) ?? [];
 
     let specifiedByURL = config.specifiedByURL;
     for (const extensionNode of extensions) {
@@ -322,7 +361,7 @@ export function extendSchemaImpl(
 
   function extendObjectType(type: GraphQLObjectType): GraphQLObjectType {
     const config = type.toConfig();
-    const extensions = typeExtensionsMap[config.name] ?? [];
+    const extensions = objectExtensions.get(config.name) ?? [];
 
     return new GraphQLObjectType({
       ...config,
@@ -342,7 +381,7 @@ export function extendSchemaImpl(
     type: GraphQLInterfaceType,
   ): GraphQLInterfaceType {
     const config = type.toConfig();
-    const extensions = typeExtensionsMap[config.name] ?? [];
+    const extensions = interfaceExtensions.get(config.name) ?? [];
 
     return new GraphQLInterfaceType({
       ...config,
@@ -360,7 +399,7 @@ export function extendSchemaImpl(
 
   function extendUnionType(type: GraphQLUnionType): GraphQLUnionType {
     const config = type.toConfig();
-    const extensions = typeExtensionsMap[config.name] ?? [];
+    const extensions = unionExtensions.get(config.name) ?? [];
 
     return new GraphQLUnionType({
       ...config,
@@ -579,10 +618,10 @@ export function extendSchemaImpl(
 
   function buildType(astNode: TypeDefinitionNode): GraphQLNamedType {
     const name = astNode.name.value;
-    const extensionASTNodes = typeExtensionsMap[name] ?? [];
 
     switch (astNode.kind) {
       case Kind.OBJECT_TYPE_DEFINITION: {
+        const extensionASTNodes = objectExtensions.get(name) ?? [];
         const allNodes = [astNode, ...extensionASTNodes];
 
         return new GraphQLObjectType({
@@ -595,6 +634,7 @@ export function extendSchemaImpl(
         });
       }
       case Kind.INTERFACE_TYPE_DEFINITION: {
+        const extensionASTNodes = interfaceExtensions.get(name) ?? [];
         const allNodes = [astNode, ...extensionASTNodes];
 
         return new GraphQLInterfaceType({
@@ -607,6 +647,7 @@ export function extendSchemaImpl(
         });
       }
       case Kind.ENUM_TYPE_DEFINITION: {
+        const extensionASTNodes = enumExtensions.get(name) ?? [];
         const allNodes = [astNode, ...extensionASTNodes];
 
         return new GraphQLEnumType({
@@ -618,6 +659,7 @@ export function extendSchemaImpl(
         });
       }
       case Kind.UNION_TYPE_DEFINITION: {
+        const extensionASTNodes = unionExtensions.get(name) ?? [];
         const allNodes = [astNode, ...extensionASTNodes];
 
         return new GraphQLUnionType({
@@ -629,6 +671,7 @@ export function extendSchemaImpl(
         });
       }
       case Kind.SCALAR_TYPE_DEFINITION: {
+        const extensionASTNodes = scalarExtensions.get(name) ?? [];
         return new GraphQLScalarType({
           name,
           description: astNode.description?.value,
@@ -638,6 +681,7 @@ export function extendSchemaImpl(
         });
       }
       case Kind.INPUT_OBJECT_TYPE_DEFINITION: {
+        const extensionASTNodes = inputObjectExtensions.get(name) ?? [];
         const allNodes = [astNode, ...extensionASTNodes];
 
         return new GraphQLInputObjectType({