Rework error handling in the service protocol and in Observatory.

runtime/observatory/lib/src/elements/script_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 script_view_element;
 
+import 'dart:async';
 import 'observatory_element.dart';
 import 'package:observatory/elements.dart';
 import 'package:observatory/service.dart';
 import 'package:polymer/polymer.dart';
 
 /// Displays an Script response.
 @CustomTag('script-view')
 class ScriptViewElement extends ObservatoryElement {
   @published Script script;
 
   ScriptViewElement.created() : super.created();
 
-  void refresh(var done) {
-    script.reload().whenComplete(done);
+  Future refresh() {
+    return script.reload();
   }
 
-  void refreshCoverage(var done) {
-    script.refreshCoverage().whenComplete(done);
+  Future refreshCoverage() {
+    return script.refreshCoverage();
   }
 }