Dart: Use dart:developer for tracing in startup benchmark.

benchmarks/dart_startup/lib/main.dart

 #!mojo mojo:dart_content_handler
 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 import 'dart:async';
+import 'dart:developer';
 
-import 'package:common/tracing_helper.dart';
 import 'package:mojo/application.dart';
 import 'package:mojo/core.dart';
 
 class StartupBenchmarkApp extends Application {
-  TracingHelper _tracing;
-  Timer _timer;
-
   StartupBenchmarkApp.fromHandle(MojoHandle handle) : super.fromHandle(handle);
 
   void initialize(List<String> args, String url) {
-    // This sets up a connection between this application and the Mojo
-    // tracing service.
-    _tracing = new TracingHelper.fromApplication(
-        this, "example_traced_application", TraceSendTiming.AT_END);
-    _tracing.traceInstant("initialized", "traced_application");
+    Timeline.instantSync("initialized");
   }
 
   @override
   void acceptConnection(String requestorUrl, String resolvedUrl,
       ApplicationConnection connection) {
-    _tracing.traceInstant("connected", "traced_application");
+    Timeline.instantSync("connected");
   }
 }
 
 main(List args) {
   MojoHandle appHandle = new MojoHandle(args[0]);
   new StartupBenchmarkApp.fromHandle(appHandle);
 }