Rework error handling in the service protocol and in Observatory.

runtime/observatory/lib/src/elements/object_common.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 object_common_element;
 
 import 'dart:async';
 import 'observatory_element.dart';
 import 'package:observatory/service.dart';
 import 'package:polymer/polymer.dart';
 
 @CustomTag('object-common')
 class ObjectCommonElement extends ObservatoryElement {
   @published ServiceObject object;
   @published ServiceMap path;
   @published ServiceMap inboundReferences;
   @observable int retainedBytes = null;
 
   ObjectCommonElement.created() : super.created();
 
   // TODO(koda): Add no-arg "calculate-link" instead of reusing "eval-link".
   Future<ServiceObject> retainedSize(var dummy) {
     return object.isolate.getRetainedSize(object).then((Instance obj) {
+      // TODO(turnidge): Handle collected/expired objects gracefully.
       retainedBytes = int.parse(obj.valueAsString);
     });
   }
 
   Future<ServiceObject> retainingPath(var limit) {
     return object.isolate.getRetainingPath(object, limit).then((ServiceObject obj) {
       path = obj;
     });
   }
 
   Future<ServiceObject> fetchInboundReferences(var limit) {
     return object.isolate.getInboundReferences(object, limit)
         .then((ServiceObject obj) {
            inboundReferences = obj;
         });
   }
 
-  void refresh(Function onDone) {
-    object.reload().whenComplete(onDone);
+  Future refresh() {
+    return object.reload();
   }
 }