Merge CommonElements and BackendHelpers!

pkg/compiler/lib/src/closure.dart

Index: pkg/compiler/lib/src/closure.dart
diff --git a/pkg/compiler/lib/src/closure.dart b/pkg/compiler/lib/src/closure.dart
index 95962b650528e20bdf3fc92db6f3015f360380ba..d2998071866ccac6bc9f56f20b587e8c34bc7098 100644
--- a/pkg/compiler/lib/src/closure.dart
+++ b/pkg/compiler/lib/src/closure.dart
@@ -223,10 +223,9 @@ class ClosureClassElement extends ClassElementX {
             // classes (since the emitter sorts classes by their id).
             compiler.idGenerator.getNextFreeId(),
             STATE_DONE) {
-    JavaScriptBackend backend = compiler.backend;
     ClassElement superclass = methodElement.isInstanceMember
-        ? backend.helpers.boundClosureClass
-        : backend.helpers.closureClass;
+        ? compiler.commonElements.boundClosureClass
+        : compiler.commonElements.closureClass;
     superclass.ensureResolved(compiler.resolution);
     supertype = superclass.thisType;
     interfaces = const Link<ResolutionDartType>();