Support user generated custom native JS classes.

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

 // Copyright (c) 2012, 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 native;
 
 import 'dart:collection' show Queue;
 
 import '../common/registry.dart' show
     Registry;
@@ skipping 14 matching lines @@
 import '../elements/modelx.dart' show
     BaseClassElementX,
     ElementX,
     FunctionElementX,
     LibraryElementX;
 import '../js/js.dart' as js;
 import '../js_backend/js_backend.dart';
 import '../js_emitter/js_emitter.dart' show
     CodeEmitterTask,
     NativeEmitter;
+import '../patch_parser.dart' show
+    checkJsInteropAnnotation;
 import '../parser/listener.dart' show
     Listener;
 import '../parser/element_listener.dart' show
     ElementListener;
 import '../parser/partial_elements.dart' show
     PartialMetadataAnnotation;
 import '../resolution/members.dart' show
     ResolverVisitor;
 import '../ssa/ssa.dart';
 import '../tokens/token.dart' show
@@ skipping 39 matching lines @@
   String quotedName = cls.nativeTagInfo;
   return quotedName.substring(1, quotedName.length - 1).split(',');
 }
 
 List<String> nativeTagsOfClass(ClassElement cls) {
   return nativeTagsOfClassRaw(cls).where((s) => !s.startsWith('!')).toList();
 }
 
 bool nativeTagsForcedNonLeaf(ClassElement cls) =>
     nativeTagsOfClassRaw(cls).contains('!nonleaf');