Make sure that tag profile updates stop after we leave the isolate view.

runtime/bin/vmservice/client/lib/src/elements/isolate_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 isolate_view_element;
 
 import 'dart:async';
 import 'observatory_element.dart';
 import 'package:observatory/app.dart';
 import 'package:observatory/service.dart';
@@ skipping 74 matching lines @@
 
   Future<ServiceObject> eval(String text) {
     return isolate.get(
         isolate.rootLib.id + "/eval?expr=${Uri.encodeComponent(text)}");
   }
 
   void _updateTagProfile() {
     isolate.updateTagProfile().then((tagProfile) {
       tagProfileChart.update(tagProfile);
       _drawTagProfileChart();
-      // Start the timer again.
-      _updateTimer = new Timer(new Duration(seconds: 1), _updateTagProfile);
+      if (_updateTimer != null) {
+        // Start the timer again.
+        _updateTimer = new Timer(new Duration(seconds: 1), _updateTagProfile);
+      }
     });
   }
 
   void enteredView() {
     super.enteredView();
     // Start a timer to update the isolate summary once a second.
     _updateTimer = new Timer(new Duration(seconds: 1), _updateTagProfile);
   }
 
   void leftView() {
     super.leftView();
     if (_updateTimer != null) {
       _updateTimer.cancel();
+      _updateTimer = null;
     }
   }
 
   void _drawTagProfileChart() {
     var element = shadowRoot.querySelector('#tagProfileChart');
     if (element != null) {
       tagProfileChart.draw(element);
     }
   }
 
@@ skipping 12 matching lines @@
 
   Future resume(_) {
     return isolate.get("resume").then((result) {
         // TODO(turnidge): Instead of asserting here, handle errors
         // properly.
         assert(result.serviceType == 'Success');
         return isolate.reload();
       });
   }
 }