Maintain allowed legacy names when extending a schema.

src/utilities/__tests__/extendSchema-test.js

@@ -994,6 +994,25 @@ describe('extendSchema', () => {
     );
   });
 
+  it('maintains configuration of the original schema object', () => {
+    const testSchemaWithLegacyNames = new GraphQLSchema({
+      query: new GraphQLObjectType({
+        name: 'Query',
+        fields: () => ({
+          id: { type: GraphQLID },
+        }),
+      }),
+      allowedLegacyNames: ['__badName'],
+    });
+    const ast = parse(`
+      extend type Query {
+        __badName: String
+      }
+    `);
+    const schema = extendSchema(testSchemaWithLegacyNames, ast);
+    expect(schema.__allowedLegacyNames).to.deep.equal(['__badName']);
+  });
+
   describe('does not allow extending a non-object type', () => {
     it('not an interface', () => {
       const ast = parse(`

src/utilities/extendSchema.js

@@ -230,6 +230,8 @@ export function extendSchema(
     types,
     directives: getMergedDirectives(),
     astNode: schema.astNode,
+    allowedLegacyNames:
+      schema.__allowedLegacyNames && schema.__allowedLegacyNames.slice(),
   });
 
   // Below are functions used for producing this schema that have closed over