Fix warnings and errors detected by dart2js.

lib/observe/list.dart

 // Copyright (c) 2013, 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 web_ui.observe.list;
 
 import 'observable.dart';
 import 'package:web_ui/src/utils_observe.dart' show Arrays, ListMixinWorkaround;
 
 /**
@@ skipping 134 matching lines @@
   void insert(int index, E item) => insertRange(index, 1, item);
 
   E removeAt(int index) {
     E result = this[index];
     removeRange(index, 1);
     return result;
   }
 
   Iterable expand(Iterable f(E)) {
     throw new UnimplementedError();
-    return null;
   }
 
   String toString() {
     if (observeReads) {
       for (int i = 0; i < length; i++) {
         notifyRead(this, ChangeRecord.INDEX, i);
       }
     }
     return _list.toString();
   }
 }