Build Observatory as part of runtime

runtime/observatory/lib/src/elements/class_tree.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 class_tree_element;
 
 import 'observatory_element.dart';
 import 'dart:html';
 import 'package:logging/logging.dart';
 import 'package:observatory/app.dart';
@@ skipping 95 matching lines @@
       try {
         // Subtract 1 to get 0 based indexing.
         tree.toggle(row.rowIndex - 1);
       }  catch (e, stackTrace) {
         Logger.root.warning('toggleExpanded', e, stackTrace);
       }
     }
   }
 
 }