Rework error handling in the service protocol and in Observatory.

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

Index: runtime/observatory/lib/src/elements/metrics.dart
diff --git a/runtime/observatory/lib/src/elements/metrics.dart b/runtime/observatory/lib/src/elements/metrics.dart
index 00b154457a2d6c9e151a207f36d779b15f88a795..b7e4e798406f25bfa0a67498bc6c416c1b21089e 100644
--- a/runtime/observatory/lib/src/elements/metrics.dart
+++ b/runtime/observatory/lib/src/elements/metrics.dart
@@ -4,6 +4,7 @@
 
 library metrics;
 
+import 'dart:async';
 import 'dart:html';
 import 'observatory_element.dart';
 import 'package:observatory/app.dart';
@@ -51,8 +52,8 @@ class MetricsPageElement extends ObservatoryElement {
     }
   }
 
-  void refresh(var done) {
-    isolate.refreshMetrics().whenComplete(done);
+  Future refresh() {
+    return isolate.refreshMetrics();
   }
 
   void selectMetric(Event e, var detail, Element target) {
@@ -195,4 +196,4 @@ class MetricsGraphElement extends ObservatoryElement {
       _setupInitialDataTable();
     }
   }
-}
+}