Rework error handling in the service protocol and in Observatory.

runtime/observatory/lib/src/elements/metrics.dart

 // Copyright (c) 2014, 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 metrics;
 
+import 'dart:async';
 import 'dart:html';
 import 'observatory_element.dart';
 import 'package:observatory/app.dart';
 import 'package:observatory/service.dart';
 import 'package:polymer/polymer.dart';
 
 @CustomTag('metrics-page')
 class MetricsPageElement extends ObservatoryElement {
   MetricsPageElement.created() : super.created();
 
@@ skipping 27 matching lines @@
   }
 
   void isolateChanged(oldValue) {
     if (isolate != null) {
       isolate.refreshMetrics().then((_) {
         _autoPickSelectedMetric();
       });
     }
   }
 
-  void refresh(var done) {
-    isolate.refreshMetrics().whenComplete(done);
+  Future refresh() {
+    return isolate.refreshMetrics();
   }
 
   void selectMetric(Event e, var detail, Element target) {
     String id = target.attributes['data-id'];
     selectedMetric = isolate.dartMetrics[id];
     if (selectedMetric == null) {
       // Check VM metrics.
       selectedMetric = isolate.nativeMetrics[id];
     }
     if (selectedMetric != null) {
@@ skipping 122 matching lines @@
       var sample = metric.samples[i];
       _table.addTimeOfDayValue(sample.time, sample.value);
     }
   }
 
   metricChanged(oldValue) {
     if (oldValue != metric) {
       _setupInitialDataTable();
     }
   }
-}
+}