Add @Native(...) annotation for native class names.

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

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library elements.modelx;
 
 import 'elements.dart';
 import '../helpers/helpers.dart';
 import '../tree/tree.dart';
 import '../util/util.dart';
@@ skipping 2251 matching lines @@
     // Use [declaration] for both [this] and [cls], because
     // declaration classes hold the superclass hierarchy.
     cls = cls.declaration;
     for (ClassElement s = declaration; s != null; s = s.superclass) {
       if (identical(s, cls)) return true;
     }
     return false;
   }
 
   bool get isNative => nativeTagInfo != null;
+
   void setNative(String name) {
+    assert(invariant(this, nativeTagInfo == null,
+        message: "Native tag info set twice on $this."));
     nativeTagInfo = name;
   }
 
   FunctionType get callType {
     MemberSignature member =
         lookupInterfaceMember(const PublicName(Compiler.CALL_OPERATOR_NAME));
     return member != null && member.isMethod ? member.type : null;
   }
 
   // TODO(johnniwinther): Remove these when issue 18630 is fixed.
@@ skipping 382 matching lines @@
   AstElement get definingElement;
 
   bool get hasResolvedAst => definingElement.hasTreeElements;
 
   ResolvedAst get resolvedAst {
     return new ResolvedAst(declaration,
         definingElement.node, definingElement.treeElements);
   }
 
 }