Support user generated custom native JS classes.

pkg/compiler/lib/src/js_backend/js_backend.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 js_backend;
 
 import 'dart:async' show EventSink, Future;
 import 'dart:collection' show HashMap;
 
 import 'package:js_runtime/shared/embedded_names.dart' as embeddedNames;
@@ skipping 54 matching lines @@
 import '../js_emitter/js_emitter.dart' show
     ClassBuilder,
     CodeEmitterTask,
     Emitter,
     MetadataCollector,
     Placeholder,
     TokenFinalizer,
     USE_LAZY_EMITTER;
 import '../library_loader.dart' show LibraryLoader, LoadedLibraries;
 import '../native/native.dart' as native;
+import '../patch_parser.dart' show
+    checkJsInteropAnnotation;
 import '../resolution/registry.dart' show
     ResolutionRegistry;
 import '../resolution/tree_elements.dart' show
     TreeElements;
 import '../ssa/ssa.dart';
 import '../tree/tree.dart';
 import '../types/types.dart';
 import '../universe/universe.dart';
 import '../util/characters.dart';
 import '../util/util.dart';
@@ skipping 10 matching lines @@
 part 'constant_handler_javascript.dart';
 part 'custom_elements_analysis.dart';
 part 'frequency_namer.dart';
 part 'field_naming_mixin.dart';
 part 'minify_namer.dart';
 part 'namer.dart';
 part 'namer_names.dart';
 part 'no_such_method_registry.dart';
 part 'runtime_types.dart';
 part 'type_variable_handler.dart';