Support user generated custom native JS classes.

pkg/compiler/lib/src/native/enqueue.dart

 // Copyright (c) 2014, 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.
 
 part of native;
 
 /**
  * This could be an abstract class but we use it as a stub for the dart_backend.
  */
 class NativeEnqueuer {
@@ skipping 524 matching lines @@
 class NativeResolutionEnqueuer extends NativeEnqueuerBase {
 
   Map<String, ClassElement> tagOwner = new Map<String, ClassElement>();
 
   NativeResolutionEnqueuer(Enqueuer world, Compiler compiler)
     : super(world, compiler, compiler.enableNativeLiveTypeAnalysis);
 
   void processNativeClass(ClassElement classElement) {
     super.processNativeClass(classElement);
 
+    // Js Interop interfaces do not have tags.
+    if (classElement.isJsInterop) return;
     // Since we map from dispatch tags to classes, a dispatch tag must be used
     // on only one native class.
     for (String tag in nativeTagsOfClass(classElement)) {
       ClassElement owner = tagOwner[tag];
       if (owner != null) {
         if (owner != classElement) {
           compiler.internalError(
               classElement, "Tag '$tag' already in use by '${owner.name}'");
         }
       } else {
@@ skipping 116 matching lines @@
         superclass,
         () => <ClassElement>[]);
     directSubtypes.add(cls);
   }
 
   void logSummary(log(message)) {
     log('Compiled ${registeredClasses.length} native classes, '
         '${unusedClasses.length} native classes omitted.');
   }
 }