Add external sizes to the heap dominator tree.

runtime/observatory/lib/src/heap_snapshot/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 heap_snapshot;
 
 class HeapSnapshot implements M.HeapSnapshot {
   ObjectGraph graph;
   DateTime timestamp;
   int get objects => graph.vertexCount;
   int get references => graph.edgeCount;
-  int get size => graph.size;
+  int get size => graph.internalSize + graph.externalSize;
   HeapSnapshotDominatorNode dominatorTree;
   HeapSnapshotMergedDominatorNode mergedDominatorTree;
   List<MergedVertex> classReferences;
 
   static Future sleep([Duration duration = const Duration(microseconds: 0)]) {
     final Completer completer = new Completer();
     new Timer(duration, () => completer.complete());
     return completer.future;
   }
 
@@ skipping 120 matching lines @@
     } else {
       return _children =
           new List.unmodifiable(v.dominatorTreeChildren().map((v) {
         return new HeapSnapshotDominatorNode(isolate, v);
       }));
     }
   }
 
   int get retainedSize => v.retainedSize;
   int get shallowSize => v.shallowSize;
+  int get externalSize => v.externalSize;
 
   HeapSnapshotDominatorNode(S.Isolate isolate, ObjectVertex vertex)
       : isolate = isolate,
         v = vertex;
 }
 
 class HeapSnapshotMergedDominatorNode
     implements M.HeapSnapshotMergedDominatorNode {
   final MergedObjectVertex v;
   final S.Isolate isolate;
@@ skipping 12 matching lines @@
       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;
+  int get externalSize => v.externalSize;
 
   HeapSnapshotMergedDominatorNode(S.Isolate isolate, MergedObjectVertex vertex)
       : isolate = isolate,
         v = vertex;
 }
 
 class MergedEdge {
   final MergedVertex sourceVertex;
   final MergedVertex targetVertex;
   int count = 0;
@@ skipping 63 matching lines @@
   final MergedEdge edge;
   S.Class get target => edge.sourceVertex != vertex
       ? edge.sourceVertex.clazz
       : edge.targetVertex.clazz;
   int get count => edge.count;
   int get shallowSize => edge.shallowSize;
   int get retainedSize => edge.retainedSize;
 
   HeapSnapshotClassOutbound(this.vertex, this.edge);
 }