Observatory: Add view of dominator tree with siblings merged by class.

runtime/observatory/lib/src/models/objects/heap_snapshot.dart

 // Copyright (c) 2016, 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.
 
 part of models;
 
 abstract class HeapSnapshot {
   DateTime get timestamp;
   int get objects;
   int get references;
   int get size;
   HeapSnapshotDominatorNode get dominatorTree;
+  HeapSnapshotMergedDominatorNode get mergedDominatorTree;
   Iterable<HeapSnapshotClassReferences> get classReferences;
 }
 
 abstract class HeapSnapshotDominatorNode {
   int get shallowSize;
   int get retainedSize;
   Future<ObjectRef> get object;
   Iterable<HeapSnapshotDominatorNode> get children;
 }
 
+abstract class HeapSnapshotMergedDominatorNode {
+  int get instanceCount;
+  int get shallowSize;
+  int get retainedSize;
+  Future<ObjectRef> get klass;
+  Iterable<HeapSnapshotMergedDominatorNode> get children;
+}
+
 abstract class HeapSnapshotClassReferences {
   ClassRef get clazz;
   int get instances;
   int get shallowSize;
   int get retainedSize;
   Iterable<HeapSnapshotClassInbound> get inbounds;
   Iterable<HeapSnapshotClassOutbound> get outbounds;
 }
 
 abstract class HeapSnapshotClassInbound {
   ClassRef get source;
   int get count;
   int get shallowSize;
   int get retainedSize;
 }
 
 abstract class HeapSnapshotClassOutbound {
   ClassRef get target;
   int get count;
   int get shallowSize;
   int get retainedSize;
 }