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

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

Index: runtime/observatory/lib/src/heap_snapshot/heap_snapshot.dart
diff --git a/runtime/observatory/lib/src/heap_snapshot/heap_snapshot.dart b/runtime/observatory/lib/src/heap_snapshot/heap_snapshot.dart
index 5345d8c1ddbb8344ea74e84dc876c8a9e0366a5c..2d7abcd313aa5b095ab199b480d75026c3f93486 100644
--- a/runtime/observatory/lib/src/heap_snapshot/heap_snapshot.dart
+++ b/runtime/observatory/lib/src/heap_snapshot/heap_snapshot.dart
@@ -11,6 +11,7 @@ class HeapSnapshot implements M.HeapSnapshot {
   int get references => graph.edgeCount;
   int get size => graph.size;
   HeapSnapshotDominatorNode dominatorTree;
+  HeapSnapshotMergedDominatorNode mergedDominatorTree;
   List<MergedVertex> classReferences;
 
   static Future sleep([Duration duration = const Duration(microseconds: 0)]) {
@@ -36,6 +37,8 @@ class HeapSnapshot implements M.HeapSnapshot {
       });
       await stream.last;
       dominatorTree = new HeapSnapshotDominatorNode(isolate, graph.root);
+      mergedDominatorTree =
+          new HeapSnapshotMergedDominatorNode(isolate, graph.mergedRoot);
       classReferences = await buildMergedVertices(isolate, graph, signal);
       progress.close();
     }());
@@ -151,6 +154,36 @@ class HeapSnapshotDominatorNode implements M.HeapSnapshotDominatorNode {
         v = vertex;
 }
 
+class HeapSnapshotMergedDominatorNode
+    implements M.HeapSnapshotMergedDominatorNode {
+  final MergedObjectVertex v;
+  final S.Isolate isolate;
+
+  Future<S.HeapObject> get klass {
+    return new Future.value(isolate.getClassByCid(v.vmCid));
+  }
+
+  Iterable<HeapSnapshotMergedDominatorNode> _children;
+  Iterable<HeapSnapshotMergedDominatorNode> get children {
+    if (_children != null) {
+      return _children;
+    } else {
+      return _children =
+          new List.unmodifiable(v.dominatorTreeChildren().map((v) {
+        return new HeapSnapshotMergedDominatorNode(isolate, v);
+      }));
+    }
+  }
+
+  int get instanceCount => v.instanceCount;
+  int get retainedSize => v.retainedSize;
+  int get shallowSize => v.shallowSize;
+
+  HeapSnapshotMergedDominatorNode(S.Isolate isolate, MergedObjectVertex vertex)
+      : isolate = isolate,
+        v = vertex;
+}
+
 class MergedEdge {
   final MergedVertex sourceVertex;
   final MergedVertex targetVertex;