Add timing prints to object group to get a baseline.

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

 // Copyright (c) 2015, 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 heap_snapshot_element;
 
 import 'dart:async';
 import 'dart:html';
 import 'observatory_element.dart';
 import 'package:observatory/app.dart';
 import 'package:observatory/service.dart';
 import 'package:observatory/elements.dart';
 import 'package:observatory/object_graph.dart';
 import 'package:polymer/polymer.dart';
+import 'package:logging/logging.dart';
 
 class DominatorTreeRow extends TableTreeRow {
   final ObjectVertex vertex;
   final HeapSnapshot snapshot;
 
   var _domTreeChildren;
   get domTreeChildren {
     if (_domTreeChildren == null) {
       _domTreeChildren = vertex.dominatorTreeChildren();
     }
@@ skipping 303 matching lines @@
   int retainedSize = 0;
 
   List<MergedEdge> incomingEdges = new List<MergedEdge>();
   Map<int, MergedEdge> outgoingEdges = new Map<int, MergedEdge>();
 
   MergedVertex(this.cid);
 }
 
 
 Future<List<MergedVertex>> buildMergedVertices(ObjectGraph graph) async {
+  Logger.root.info("Start merge vertices");
+
   var cidToMergedVertex = {};
 
   for (var vertex in graph.vertices) {
     var cid = vertex.vmCid;
     MergedVertex source = cidToMergedVertex[cid];
     if (source == null) {
       cidToMergedVertex[cid] = source = new MergedVertex(cid);
     }
 
     source.instances++;
@@ skipping 10 matching lines @@
         edge = new MergedEdge(source, target);
         source.outgoingEdges[cid2] = edge;
         target.incomingEdges.add(edge);
       }
       edge.count++;
       // An over-estimate if there are multiple references to the same object.
       edge.shallowSize += vertex2.shallowSize == null ? 0 : vertex2.shallowSize;
     }
   }
 
+  Logger.root.info("End merge vertices");
+
   return cidToMergedVertex.values.toList();
 }
 
 @CustomTag('heap-snapshot')
 class HeapSnapshotElement extends ObservatoryElement {
   @published Isolate isolate;
   @observable HeapSnapshot snapshot;
 
   @published String state = 'Requested';
   @published String analysisSelector = 'DominatorTree';
@@ skipping 73 matching lines @@
 
     new Future.delayed(const Duration(milliseconds: 500), () {
       buildMergedVertices(snapshot.graph).then((vertices) {
         state = 'Loaded';
         var rootRow = new MergedVerticesRow(tree, null, isolate, vertices);
         tree.initialize(rootRow);
       });
     });
   }
 }