Use entities in CommonElements interface.

pkg/compiler/lib/src/js_backend/backend.dart

 // Copyright (c) 2012, 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 js_backend.backend;
 
 import 'dart:async' show Future;
 
 import 'package:js_runtime/shared/embedded_names.dart' as embeddedNames;
 
@@ skipping 963 matching lines @@
   }
 
   bool operatorEqHandlesNullArgument(FunctionElement operatorEqfunction) {
     return specialOperatorEqClasses.contains(operatorEqfunction.enclosingClass);
   }
 
   void validateInterceptorImplementsAllObjectMethods(
       ClassElement interceptorClass) {
     if (interceptorClass == null) return;
     interceptorClass.ensureResolved(resolution);
-    commonElements.objectClass.forEachMember((_, Element member) {
+    ClassElement objectClass = commonElements.objectClass;
+    objectClass.forEachMember((_, Element member) {
       if (member.isGenerativeConstructor) return;
       Element interceptorMember = interceptorClass.lookupMember(member.name);
       // Interceptors must override all Object methods due to calling convention
       // differences.
       assert(invariant(interceptorMember,
           interceptorMember.enclosingClass == interceptorClass,
           message:
               "Member ${member.name} not overridden in ${interceptorClass}. "
               "Found $interceptorMember from "
               "${interceptorMember.enclosingClass}."));
@@ skipping 2305 matching lines @@
   @override
   bool isNativeClass(ClassElement element) {
     return helpers.backend.isNative(element);
   }
 
   @override
   bool isNativeMember(MemberElement element) {
     return helpers.backend.isNative(element);
   }
 }