dart2js: Fix lazy statics in the startup-emitter.

pkg/compiler/lib/src/js_emitter/startup_emitter/fragment_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.
 
 part of dart2js.js_emitter.startup_emitter.model_emitter;
 
 /// The name of the property that stores the tear-off getter on a static
 /// function.
 ///
 /// This property is only used when isolates are used.
@@ skipping 116 matching lines @@
 function mixin(cls, mixin) {
   copyProperties(mixin.prototype, cls.prototype);
 }
 
 // Creates a lazy field.
 //
 // A lazy field has a storage entry, [name], which holds the value, and a
 // getter ([getterName]) to access the field. If the field wasn't set before
 // the first access, it is initialized with the [initializer].
 function lazy(holder, name, getterName, initializer) {
-  holder[name] = null;
+  var uninitializedSentinel = holder;
+  holder[name] = uninitializedSentinel;
   holder[getterName] = function() {
     holder[getterName] = function() { #cyclicThrow(name) };
     var result;
     var sentinelInProgress = initializer;
     try {
-      result = holder[name] = sentinelInProgress;
-      result = holder[name] = initializer();
+      if (holder[name] === uninitializedSentinel) {
+        result = holder[name] = sentinelInProgress;
+        result = holder[name] = initializer();
+      } else {
+        result = holder[name];
+      }
     } finally {
       // Use try-finally, not try-catch/throw as it destroys the stack
       // trace.
       if (result === sentinelInProgress) {
         // The lazy static (holder[name]) might have been set to a different
         // value. According to spec we still have to reset it to null, if
         // the initialization failed.
         holder[name] = null;
       }
       // TODO(floitsch): for performance reasons the function should probably
@@ skipping 1132 matching lines @@
     }
     statements.add(js.js.statement("setOrUpdateInterceptorsByTag(#);",
         js.objectLiteral(interceptorsByTag)));
     statements.add(js.js.statement("setOrUpdateLeafTags(#);",
         js.objectLiteral(leafTags)));
     statements.add(subclassAssignment);
 
     return new js.Block(statements);
   }
 }