Ensure that collectedClasses always contains a [globalObject, descriptor] pair.

pkg/dart2js_incremental/lib/library_updater.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.
 
 library dart2js_incremental.library_updater;
 
 import 'dart:async' show
     Future;
 
 import 'dart:convert' show
@@ skipping 910 matching lines @@
   }
 
   jsAst.Expression invokeDefineClass(ClassElementX cls) {
     String name = namer.className(cls);
     var descriptor = js('Object.create(null)');
     return js(
         r'''
 (new Function(
     "$collectedClasses", "$desc",
     this.defineClass(#name, #computeFields) +"\n;return " + #name))(
-        {#name: #descriptor})''',
+        {#name: [,#descriptor]})''',
         {'name': js.string(name),
          'computeFields': js.stringArray(computeFields(cls)),
          'descriptor': descriptor});
   }
 
   jsAst.Node computeMethodUpdateJs(Element element) {
     Method member = new ProgramBuilder(compiler, namer, emitter)
         .buildMethodHackForIncrementalCompilation(element);
     if (member == null) {
       compiler.internalError(element, '${element.runtimeType}');
@@ skipping 545 matching lines @@
         .buildFieldsHackForIncrementalCompilation(classElement);
     // TODO(ahe): Rewrite for new emitter.
     ClassBuilder builder = new ClassBuilder(classElement, namer);
     classEmitter.emitFields(cls, builder);
     return builder.fields;
   }
 }
 
 // TODO(ahe): Remove this method.
 NO_WARN(x) => x;