Another attempt at reversion.

src/utilities/buildClientSchema.js

@@ -7,9 +7,9 @@
  * @flow strict
  */
 
-import objectValues from '../polyfills/objectValues';
 import inspect from '../jsutils/inspect';
 import invariant from '../jsutils/invariant';
+import keyMap from '../jsutils/keyMap';
 import keyValMap from '../jsutils/keyValMap';
 import { valueFromAST } from './valueFromAST';
 import { parseValue } from '../language/parser';
@@ -84,18 +84,25 @@ export function buildClientSchema(
   // Get the schema from the introspection result.
   const schemaIntrospection = introspection.__schema;
 
-  // Iterate through all types, getting the type definition for each.
-  const typeMap = keyValMap(
+  // Converts the list of types into a keyMap based on the type names.
+  const typeIntrospectionMap = keyMap(
     schemaIntrospection.types,
-    typeIntrospection => typeIntrospection.name,
-    typeIntrospection => buildType(typeIntrospection),
+    type => type.name,
   );
 
-  for (const stdType of [...specifiedScalarTypes, ...introspectionTypes]) {
-    if (typeMap[stdType.name]) {
-      typeMap[stdType.name] = stdType;
-    }
-  }
+  // A cache to use to store the actual GraphQLType definition objects by name.
+  // Initialize to the GraphQL built in scalars. All functions below are inline
+  // so that this type def cache is within the scope of the closure.
+  const typeDefCache = keyMap(
+    specifiedScalarTypes.concat(introspectionTypes),
+    type => type.name,
+  );
+
+  // Iterate through all types, getting the type definition for each, ensuring
+  // that any type not directly referenced by a field will get created.
+  const types = schemaIntrospection.types.map(typeIntrospection =>
+    getNamedType(typeIntrospection.name),
+  );
 
   // Get the root Query, Mutation, and Subscription types.
   const queryType = schemaIntrospection.queryType
@@ -121,7 +128,7 @@ export function buildClientSchema(
     query: queryType,
     mutation: mutationType,
     subscription: subscriptionType,
-    types: objectValues(typeMap),
+    types,
     directives,
     assumeValid: options && options.assumeValid,
     allowedLegacyNames: options && options.allowedLegacyNames,
@@ -152,16 +159,20 @@ export function buildClientSchema(
   }
 
   function getNamedType(typeName: string): GraphQLNamedType {
-    const type = typeMap[typeName];
-    if (!type) {
+    if (typeDefCache[typeName]) {
+      return typeDefCache[typeName];
+    }
+    const typeIntrospection = typeIntrospectionMap[typeName];
+    if (!typeIntrospection) {
       throw new Error(
         `Invalid or incomplete schema, unknown type: ${typeName}. Ensure ` +
           'that a full introspection query is used in order to build a ' +
           'client schema.',
       );
     }
-
-    return type;
+    const typeDef = buildType(typeIntrospection);
+    typeDefCache[typeName] = typeDef;
+    return typeDef;
   }
 
   function getInputType(typeRef: IntrospectionInputTypeRef): GraphQLInputType {