Revert "Build Observatory with runtime"

runtime/bin/vmservice/observatory/lib/object_graph.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 object_graph;
 
 import 'dart:typed_data';
 
 import 'dominator_tree.dart';
 
@@ skipping 100 matching lines @@
       var v = leaves.removeLast();
       var u = d.dominator(v);
       if (u == null) continue;
       u._retainedSize += v._retainedSize;
       if (--degree[u] == 0) {
         leaves.add(u);
       }
     }
   }
 }