Use entities in CommonElements interface.

tests/compiler/dart2js/mock_compiler.dart

 // Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library mock_compiler;
 
 import 'dart:async';
 import 'dart:collection';
 
 import 'package:compiler/compiler.dart' as api;
@@ skipping 131 matching lines @@
   /// Initialize the mock compiler with an empty main library.
   Future<Uri> init([String mainSource = ""]) {
     Uri uri = new Uri(scheme: "mock");
     registerSource(uri, mainSource);
     return libraryLoader.loadLibrary(uri).then((LibraryElement library) {
       mainApp = library;
       // We need to make sure the Object class is resolved. When registering a
       // dynamic invocation the ArgumentTypesRegistry eventually iterates over
       // the interfaces of the Object class which would be 'null' if the class
       // wasn't resolved.
-      commonElements.objectClass.ensureResolved(resolution);
+      ClassElement objectClass = commonElements.objectClass;
+      objectClass.ensureResolved(resolution);
     }).then((_) => uri);
   }
 
   Future run(Uri uri, [String mainSource = ""]) {
     return init(mainSource).then((Uri mainUri) {
       return super.run(uri == null ? mainUri : uri);
     }).then((result) {
       if (expectedErrors != null &&
           expectedErrors != diagnosticCollector.errors.length) {
         throw "unexpected error during compilation "
@@ skipping 228 matching lines @@
       trustTypeAnnotations: trustTypeAnnotations,
       enableTypeAssertions: enableTypeAssertions,
       enableUserAssertions: enableUserAssertions,
       expectedErrors: expectedErrors,
       expectedWarnings: expectedWarnings,
       outputProvider: outputProvider);
   compiler.registerSource(uri, code);
   compiler.diagnosticHandler = createHandler(compiler, code);
   return compiler;
 }