Rework error handling in the service protocol and in Observatory.

runtime/observatory/lib/src/elements/flag_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 flag_list_element;
 
+import 'dart:async';
 import 'package:polymer/polymer.dart';
 import 'observatory_element.dart';
 import 'package:observatory/service.dart';
 
 @CustomTag('flag-list')
 class FlagListElement extends ObservatoryElement {
   @published ServiceMap flagList;
 
   FlagListElement.created() : super.created();
 
-  void refresh(var done) {
-    flagList.reload().whenComplete(done);
+  Future refresh() {
+    return flagList.reload();
   }
 }
 
 @CustomTag('flag-item')
 class FlagItemElement extends ObservatoryElement {
   @published ObservableMap flag;
 
   FlagItemElement.created() : super.created();
 }