dart2js: Add an empty "MANGLED_NAMES" embedded global to the startup-emitter.

pkg/compiler/lib/src/js_emitter/startup_emitter/model_emitter.dart

 // Copyright (c) 2015, 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 dart2js.js_emitter.startup_emitter.model_emitter;
 
 import 'dart:convert' show JsonEncoder;
 
 import '../../common.dart';
 
@@ skipping 30 matching lines @@
     DEFERRED_INITIALIZED,
     DEFERRED_LIBRARY_URIS,
     DEFERRED_LIBRARY_HASHES,
     GET_TYPE_FROM_NAME,
     INITIALIZE_LOADED_HUNK,
     INTERCEPTORS_BY_TAG,
     IS_HUNK_INITIALIZED,
     IS_HUNK_LOADED,
     LEAF_TAGS,
     MANGLED_GLOBAL_NAMES,
+    MANGLED_NAMES,
     METADATA,
     NATIVE_SUPERCLASS_TAG_NAME,
     TYPE_TO_INTERCEPTOR_MAP,
     TYPES;
 
 import '../js_emitter.dart' show NativeGenerator, buildTearOffCode;
 import '../model.dart';
 
 part 'deferred_fragment_hash.dart';
 part 'fragment_emitter.dart';
@@ skipping 322 matching lines @@
     // Json does not support comments, so we embed the explanation in the
     // data.
     mapping["_comment"] = "This mapping shows which compiled `.js` files are "
         "needed for a given deferred library import.";
     mapping.addAll(compiler.deferredLoadTask.computeDeferredMap());
     compiler.outputProvider(compiler.deferredMapUri.path, 'deferred_map')
       ..add(const JsonEncoder.withIndent("  ").convert(mapping))
       ..close();
   }
 }