Add FrontendStrategy.commonElements and remove Backend.commonElements

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

Index: pkg/compiler/lib/src/js_backend/element_strategy.dart
diff --git a/pkg/compiler/lib/src/js_backend/element_strategy.dart b/pkg/compiler/lib/src/js_backend/element_strategy.dart
index 9543a11116509f1e384ac801a420dbda61b21287..b0c216f17fdf755ac98c872cb273d4cf26a3399a 100644
--- a/pkg/compiler/lib/src/js_backend/element_strategy.dart
+++ b/pkg/compiler/lib/src/js_backend/element_strategy.dart
@@ -114,7 +114,7 @@ class ElementCodegenWorkItemBuilder extends WorkItemBuilder {
   WorkItem createWorkItem(MemberElement element) {
     assert(element.isDeclaration, failedAt(element));
     // Don't generate code for foreign elements.
-    if (_backend.isForeign(element)) return null;
+    if (_backend.isForeign(_closedWorld.commonElements, element)) return null;
     if (element.isAbstract) return null;
 
     // Codegen inlines field initializers. It only needs to generate