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

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

Index: runtime/observatory/lib/src/models/objects/heap_snapshot.dart
diff --git a/runtime/observatory/lib/src/models/objects/heap_snapshot.dart b/runtime/observatory/lib/src/models/objects/heap_snapshot.dart
index 19392f9eb326f43a1878561cc7a1cec63b470dd1..ff4cefeccd9636a7fe9a63769f8408b9a4c299c3 100644
--- a/runtime/observatory/lib/src/models/objects/heap_snapshot.dart
+++ b/runtime/observatory/lib/src/models/objects/heap_snapshot.dart
@@ -10,6 +10,7 @@ abstract class HeapSnapshot {
   int get references;
   int get size;
   HeapSnapshotDominatorNode get dominatorTree;
+  HeapSnapshotMergedDominatorNode get mergedDominatorTree;
   Iterable<HeapSnapshotClassReferences> get classReferences;
 }
 
@@ -20,6 +21,14 @@ abstract class HeapSnapshotDominatorNode {
   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;