Fix heap map

runtime/observatory/lib/src/elements/heap_map.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 heap_map_element;
 
 import 'dart:async';
 import 'dart:html';
 import 'dart:math';
 import 'observatory_element.dart';
@@ skipping 33 matching lines @@
 }
 
 class ObjectInfo {
   final address;
   final size;
   ObjectInfo(this.address, this.size);
 }
 
 @CustomTag('heap-map')
 class HeapMapElement extends ObservatoryElement {
-  var _fragmentationCanvas;
+  CanvasElement _fragmentationCanvas;
   var _fragmentationData;
   var _pageHeight;
   var _classIdToColor = {};
   var _colorToClassId = {};
   var _classIdToName = {};
 
   static final _freeColor = [255, 255, 255, 255];
   static final _pageSeparationColor = [0, 0, 0, 255];
   static const _PAGE_SEPARATION_HEIGHT = 4;
   // Many browsers will not display a very tall canvas.
@@ skipping 53 matching lines @@
     var rng = new Random(classId);
     return [rng.nextInt(128), rng.nextInt(128), rng.nextInt(128), 255];
   }
 
   String _classNameAt(Point<int> point) {
     var color = new PixelReference(_fragmentationData, point).color;
     return _classIdToName[_colorToClassId[_packColor(color)]];
   }
 
   ObjectInfo _objectAt(Point<int> point) {
+    if (fragmentation == null || _fragmentationCanvas == null) {
+      return null;
+    }
     var pagePixels = _pageHeight * _fragmentationData.width;
     var index = new PixelReference(_fragmentationData, point).index;
     var pageIndex = index ~/ pagePixels;
     var pageOffset = index % pagePixels;
     var pages = fragmentation['pages'];
     if (pageIndex < 0 || pageIndex >= pages.length) {
       return null;
     }
     // Scan the page to find start and size.
     var page = pages[pageIndex];
     var objects = page['objects'];
     var offset = 0;
     var size = 0;
     for (var i = 0; i < objects.length; i += 2) {
       size = objects[i];
       offset += size;
       if (offset > pageOffset) {
         pageOffset = offset - size;
         break;
       }
     }
     return new ObjectInfo(int.parse(page['objectStart']) +
                           pageOffset * fragmentation['unitSizeBytes'],
         size * fragmentation['unitSizeBytes']);
   }
 
   void _handleMouseMove(MouseEvent event) {
     var info = _objectAt(event.offset);
+    if (info == null) {
+      status = '';
+      return;
+    }
     var addressString = '${info.size}B @ 0x${info.address.toRadixString(16)}';
     var className = _classNameAt(event.offset);
     status = (className == '') ? '-' : '$className $addressString';
   }
 
   void _handleClick(MouseEvent event) {
     var address = _objectAt(event.offset).address.toRadixString(16);
     isolate.getObjectByAddress(address).then((result) {
       if (result.type != 'Sentinel') {
         app.locationManager.go(gotoLink('/inspect', result));
@@ skipping 76 matching lines @@
     });
   }
 
   void fragmentationChanged(oldValue) {
     // Async, in case attached has not yet run (observed in JS version).
     new Future(() {
       _updateFragmentationData();
     });
   }
 }