Add @JSName annotation for native fields and methods.

sdk/lib/_internal/compiler/implementation/elements/elements.dart

Index: sdk/lib/_internal/compiler/implementation/elements/elements.dart
diff --git a/sdk/lib/_internal/compiler/implementation/elements/elements.dart b/sdk/lib/_internal/compiler/implementation/elements/elements.dart
index df07e95ef89fddb49fd694ded8c548b74f16c6fb..921a6af407cf307cebf5af832fe0edf22f66f166 100644
--- a/sdk/lib/_internal/compiler/implementation/elements/elements.dart
+++ b/sdk/lib/_internal/compiler/implementation/elements/elements.dart
@@ -355,9 +355,11 @@ class Element implements Spannable {
     }
   }
 
-  bool _isNative = false;
-  void setNative() { _isNative = true; }
-  bool isNative() => _isNative;
+  String _nativeName = null;
+  bool isNative() => _nativeName != null;
+  String nativeName() => _nativeName;
+  /// Marks this element as a native element.
+  void setNative(String name) { _nativeName = name; }
 
   FunctionElement asFunctionElement() => null;
 
@@ -1387,7 +1389,7 @@ abstract class ClassElement extends ScopeContainerElement
   DartType supertype;
   DartType defaultClass;
   Link<DartType> interfaces;
-  SourceString nativeName;
+  SourceString nativeTagInfo;
   int supertypeLoadState;
   int resolutionState;
 
@@ -1724,7 +1726,7 @@ abstract class ClassElement extends ScopeContainerElement
   }
 
   bool isInterface() => false;
-  bool isNative() => nativeName != null;
+  bool isNative() => nativeTagInfo != null;
   int get hashCode => id;
 
   Scope buildScope() => new ClassScope(enclosingElement.buildScope(), this);