Fix exception on first navigation to metrics page

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();
 
   @observable MetricsPage page;
   @observable Isolate isolate;
   @observable ServiceMetric selectedMetric;
 
   void _autoPickSelectedMetric() {
     if (selectedMetric != null) {
       return;
     }
     // Attempt to pick the last selected metric.
     if ((isolate != null) && (page != null) &&
         (page.selectedMetricId != null)) {
       selectedMetric = isolate.dartMetrics[page.selectedMetricId];
-      if (selectedMetric == null) {
-        selectedMetric = isolate.nativeMetrics[page.selectedMetricId];
+      if (selectedMetric != null) {
+        return;
       }
+      selectedMetric = isolate.nativeMetrics[page.selectedMetricId];
     }
     if ((selectedMetric == null) && (isolate != null)) {
-      var values = isolate.dartMetrics.values;
-      if (values != null) {
+      var values = isolate.dartMetrics.values.toList();
+      if ((values != null) && (values.length > 0)) {
         // Fall back and pick the first isolate metric.
         selectedMetric = values.first;
       }
+      if (selectedMetric != null) {
+        return;
+      }
+      values = isolate.nativeMetrics.values.toList();
+      if ((values != null) && (values.length > 0)) {
+        // Fall back and pick the first isolate metric.
+        selectedMetric = values.first;
+      }
     }
   }
 
   void attached() {
     _autoPickSelectedMetric();
   }
 
   void isolateChanged(oldValue) {
     if (isolate != null) {
       isolate.refreshMetrics().then((_) {
@@ skipping 140 matching lines @@
       _table.addTimeOfDayValue(sample.time, sample.value);
     }
   }
 
   metricChanged(oldValue) {
     if (oldValue != metric) {
       _setupInitialDataTable();
     }
   }
 }