Merge CommonElements and BackendHelpers!

pkg/compiler/lib/src/inferrer/map_tracer.dart

 // Copyright (c) 2014, 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 compiler.src.inferrer.map_tracer;
 
 import '../elements/elements.dart';
-import '../js_backend/backend_helpers.dart';
+import '../js_backend/backend.dart' show JavaScriptBackend;
 import '../universe/selector.dart' show Selector;
 import 'node_tracer.dart';
 import 'type_graph_nodes.dart';
 
 Set<String> okMapSelectorsSet = new Set.from(const <String>[
   // From Object.
   "==",
   "hashCode",
   "toString",
   "noSuchMethod",
@@ skipping 42 matching lines @@
   }
 
   visitClosureCallSiteTypeInformation(ClosureCallSiteTypeInformation info) {
     bailout('Passed to a closure');
   }
 
   visitStaticCallSiteTypeInformation(StaticCallSiteTypeInformation info) {
     super.visitStaticCallSiteTypeInformation(info);
     Element called = info.calledElement;
     if (compiler.backend.isForeign(called) &&
-        called.name == BackendHelpers.JS) {
+        called.name == JavaScriptBackend.JS) {
       bailout('Used in JS ${info.call}');
     }
   }
 
   visitDynamicCallSiteTypeInformation(DynamicCallSiteTypeInformation info) {
     super.visitDynamicCallSiteTypeInformation(info);
     Selector selector = info.selector;
     String selectorName = selector.name;
     if (currentUser == info.receiver) {
       if (!okMapSelectorsSet.contains(selectorName)) {
@@ skipping 39 matching lines @@
           return;
         }
       }
     } else if (selector.isCall &&
         !info.targets.every((element) => element.isFunction)) {
       bailout('Passed to a closure');
       return;
     }
   }
 }