Fix bad isolate summary charts

runtime/observatory/lib/src/elements/isolate_summary.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_summary_element;
 
 import 'dart:html';
 import 'observatory_element.dart';
 import 'package:charted/charted.dart';
 import "package:charted/charts/charts.dart";
@@ skipping 79 matching lines @@
 
   @published ObservableMap counters;
   CounterChart chart;
 
   attached() {
     super.attached();
     chart =
         new CounterChart(shadowRoot.querySelector('#isolate-counter-chart'));
   }
 
+  detached() {
+    super.detached();
+    var host = shadowRoot.querySelector('#isolate-counter-chart-host');
+    host.children.clear();
+    var legendHost =
+        shadowRoot.querySelector('#isolate-counter-chart-legend-host');
+    legendHost.children.clear();
+  }
+
   void countersChanged(oldValue) {
     if (counters == null) {
       return;
     }
     if (chart == null) {
       return;
     }
     chart.update(counters);
   }
 }