dart2js: Avoid Object.create for bookeeping data structures during startup.

pkg/compiler/lib/src/js_emitter/old_emitter/emitter.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 dart2js.js_emitter;
 
 
 class OldEmitter implements Emitter {
   final Compiler compiler;
   final CodeEmitterTask task;
@@ skipping 779 matching lines @@
     jsAst.Expression finishedClassesAccess =
         generateEmbeddedGlobalAccess(embeddedNames.FINISHED_CLASSES);
     jsAst.Expression cyclicThrow =
         staticFunctionAccess(backend.getCyclicThrowHelper());
     jsAst.Expression laziesAccess =
         generateEmbeddedGlobalAccess(embeddedNames.LAZIES);
 
     return js.statement('''
       function init() {
         $isolatePropertiesName = Object.create(null);
-        #allClasses = Object.create(null);
+        #allClasses = map();
         #getTypeFromName = function(name) {return #allClasses[name];};
-        #interceptorsByTag = Object.create(null);
-        #leafTags = Object.create(null);
-        #finishedClasses = Object.create(null);
+        #interceptorsByTag = map();
+        #leafTags = map();
+        #finishedClasses = map();
 
         if (#needsLazyInitializer) {
           // [staticName] is only provided in non-minified mode. If missing, we 
           // fall back to [fieldName]. Likewise, [prototype] is optional and 
           // defaults to the isolateProperties object.
           $lazyInitializerName = function (fieldName, getterName, lazyValue,
                                            staticName, prototype) {
             if (!#lazies) #lazies = Object.create(null);
             #lazies[fieldName] = getterName;
 
@@ skipping 1187 matching lines @@
     for (Element element in compiler.enqueuer.codegen.newlyEnqueuedElements) {
       if (element.isInstanceMember) {
         cachedClassBuilders.remove(element.enclosingClass);
 
         nativeEmitter.cachedBuilders.remove(element.enclosingClass);
 
       }
     }
   }
 }