Observatory:

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 'package:observatory/models.dart' as M;
@@ skipping 80 matching lines @@
   S.ServiceMap _fragmentation;
 
   void render() {
     if (_canvas == null) {
       _canvas = new CanvasElement()
         ..width = 1
         ..height = 1
         ..onMouseMove.listen(_handleMouseMove)
         ..onMouseDown.listen(_handleClick);
     }
+
+    // Set hover text to describe the object under the cursor.
+    _canvas.title = _status;
+
     children = [
       navBar([
         new NavTopMenuElement(queue: _r.queue),
         new NavVMMenuElement(_vm, _events, queue: _r.queue),
         new NavIsolateMenuElement(_isolate, _events, queue: _r.queue),
         navMenu('heap map'),
         new NavRefreshElement(queue: _r.queue)
           ..onRefresh.listen((_) => _refresh()),
         new NavNotifyElement(_notifications, queue: _r.queue)
       ]),
@@ skipping 199 matching lines @@
 
   // The row-major index of this pixel.
   int get index => _dataIndex ~/ NUM_COLOR_COMPONENTS;
 }
 
 class ObjectInfo {
   final address;
   final size;
   ObjectInfo(this.address, this.size);
 }