Fix dart2js warnings and add test to ensure it stays clean.

pkg/compiler/lib/src/serialization/modelz.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.
 
 /// Implementation of the element model used for deserialiation.
 ///
 /// These classes are created by [ElementDeserializer] triggered by the
 /// [Deserializer].
 
 library dart2js.serialization.modelz;
@@ skipping 2380 matching lines @@
   final Element annotatedElement;
   final SourceSpan sourcePosition;
   final ConstantExpression constant;
 
   MetadataAnnotationZ(
       this.annotatedElement, this.sourcePosition, this.constant);
 
   @override
   MetadataAnnotation ensureResolved(Resolution resolution) {
     // Do nothing.
+    return this;
   }
 
   @override
   Node get node => throw new UnsupportedError('${this}.node');
 
   @override
   bool get hasNode => false;
 
   String toString() => 'MetadataAnnotationZ(${constant.toDartText()})';
 }