Redo LocationManager to use HTML5 History API

runtime/bin/vmservice/client/lib/src/elements/isolate_view.dart

Index: runtime/bin/vmservice/client/lib/src/elements/isolate_view.dart
diff --git a/runtime/bin/vmservice/client/lib/src/elements/isolate_view.dart b/runtime/bin/vmservice/client/lib/src/elements/isolate_view.dart
index 582c9eef83c37135ee1941f57813b471cf670327..c55b1d1adeeb42f8b9e854cada076dbe4ee30ab6 100644
--- a/runtime/bin/vmservice/client/lib/src/elements/isolate_view.dart
+++ b/runtime/bin/vmservice/client/lib/src/elements/isolate_view.dart
@@ -92,8 +92,10 @@ class IsolateViewElement extends ObservatoryElement {
     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);
+      }
     });
   }
 
@@ -107,6 +109,7 @@ class IsolateViewElement extends ObservatoryElement {
     super.leftView();
     if (_updateTimer != null) {
       _updateTimer.cancel();
+      _updateTimer = null;
     }
   }