Add @JSName annotation for native fields and methods.

sdk/lib/_internal/compiler/implementation/js_backend/emitter.dart

Index: sdk/lib/_internal/compiler/implementation/js_backend/emitter.dart
diff --git a/sdk/lib/_internal/compiler/implementation/js_backend/emitter.dart b/sdk/lib/_internal/compiler/implementation/js_backend/emitter.dart
index 0055ba20eda2dcdff45a57b3409f83b7ce296973..f1cd9791094191663ee2a3749d8ec49e798b714f 100644
--- a/sdk/lib/_internal/compiler/implementation/js_backend/emitter.dart
+++ b/sdk/lib/_internal/compiler/implementation/js_backend/emitter.dart
@@ -731,7 +731,6 @@ $lazyInitializerLogic
         compiler.codegenWorld.instantiatedClasses.contains(classElement);
 
     void visitField(ClassElement enclosingClass, Element member) {
-      assert(!member.isNative());
       assert(invariant(classElement, member.isDeclaration));
 
       LibraryElement library = member.getLibrary();
@@ -756,9 +755,12 @@ $lazyInitializerLogic
       if ((isInstantiated && !enclosingClass.isNative())
           || needsGetter
           || needsSetter) {
-        String fieldName = isShadowed
+        String accessorName = isShadowed
             ? namer.shadowedFieldName(member)
             : namer.getName(member);
+        String fieldName = member.isNative()
+            ? member.nativeName()
+            : accessorName;
         bool needsCheckedSetter = false;
         if (needsSetter && compiler.enableTypeAssertions
             && canGenerateCheckedSetter(member)) {
@@ -1253,7 +1255,9 @@ $classesCollector.$mangledName = {'':
     if (member.isGetter()) {
       getter = "this.${namer.getterName(member.getLibrary(), member.name)}()";
     } else {
-      String name = namer.instanceFieldName(memberLibrary, member.name);
+      String name = member.isNative()
+          ? member.nativeName()
+          : namer.instanceFieldName(memberLibrary, member.name);
       getter = "this.$name";
     }
     for (Selector selector in selectors) {