Rework error handling in the service protocol and in Observatory.

runtime/observatory/lib/src/elements/instance_view.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 instance_view_element;
 
 import 'dart:async';
 import 'observatory_element.dart';
 import 'package:observatory/service.dart';
 import 'package:polymer/polymer.dart';
 
 @CustomTag('instance-view')
 class InstanceViewElement extends ObservatoryElement {
   @published Instance instance;
 
   InstanceViewElement.created() : super.created();
 
   Future<ServiceObject> eval(String expression) {
     return instance.isolate.eval(instance, expression);
   }
 
-  void refresh(Function onDone) {
-    instance.reload().whenComplete(onDone);
+  Future refresh() {
+    return instance.reload();
   }
 }