Add timing prints to object group to get a baseline.

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

Index: runtime/observatory/lib/src/elements/heap_snapshot.dart
diff --git a/runtime/observatory/lib/src/elements/heap_snapshot.dart b/runtime/observatory/lib/src/elements/heap_snapshot.dart
index 1dee104e9242e2ebf921e2fea41dba39082bda34..a9e4a8ef2d8941889ef18ed861dd2dc95f8e17cc 100644
--- a/runtime/observatory/lib/src/elements/heap_snapshot.dart
+++ b/runtime/observatory/lib/src/elements/heap_snapshot.dart
@@ -12,6 +12,7 @@ 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;
@@ -335,6 +336,8 @@ class MergedVertex {
 
 
 Future<List<MergedVertex>> buildMergedVertices(ObjectGraph graph) async {
+  Logger.root.info("Start merge vertices");
+
   var cidToMergedVertex = {};
 
   for (var vertex in graph.vertices) {
@@ -365,6 +368,8 @@ Future<List<MergedVertex>> buildMergedVertices(ObjectGraph graph) async {
     }
   }
 
+  Logger.root.info("End merge vertices");
+
   return cidToMergedVertex.values.toList();
 }