Merge CommonElements and BackendHelpers!

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

Index: pkg/compiler/lib/src/inferrer/closure_tracer.dart
diff --git a/pkg/compiler/lib/src/inferrer/closure_tracer.dart b/pkg/compiler/lib/src/inferrer/closure_tracer.dart
index d3157ef5978fca01856f05e8a7a0a378415e042d..1f9bb63d05601202d5fdfbc77ec0f33564e01161 100644
--- a/pkg/compiler/lib/src/inferrer/closure_tracer.dart
+++ b/pkg/compiler/lib/src/inferrer/closure_tracer.dart
@@ -6,7 +6,7 @@ library compiler.src.inferrer.closure_tracer;
 
 import '../common/names.dart' show Names;
 import '../elements/elements.dart';
-import '../js_backend/backend_helpers.dart';
+import '../js_backend/backend.dart' show JavaScriptBackend;
 import '../types/types.dart' show TypeMask;
 import '../universe/selector.dart' show Selector;
 import 'debug.dart' as debug;
@@ -79,7 +79,7 @@ class ClosureTracerVisitor extends TracerVisitor {
     Element called = info.calledElement;
     if (compiler.backend.isForeign(called)) {
       String name = called.name;
-      if (name == BackendHelpers.JS || name == 'DART_CLOSURE_TO_JS') {
+      if (name == JavaScriptBackend.JS || name == 'DART_CLOSURE_TO_JS') {
         bailout('Used in JS ${info.call}');
       }
     }