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

pkg/compiler/lib/src/js_emitter/old_emitter/emitter.dart

Index: pkg/compiler/lib/src/js_emitter/old_emitter/emitter.dart
diff --git a/pkg/compiler/lib/src/js_emitter/old_emitter/emitter.dart b/pkg/compiler/lib/src/js_emitter/old_emitter/emitter.dart
index 06f080341a464cbce307e5454dfd4bb68e003f8e..334b7462e8518239135b2a111047fc190f7cd8e4 100644
--- a/pkg/compiler/lib/src/js_emitter/old_emitter/emitter.dart
+++ b/pkg/compiler/lib/src/js_emitter/old_emitter/emitter.dart
@@ -797,11 +797,11 @@ class OldEmitter implements Emitter {
     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