Add tracing event for first connection.

examples/dart/traced_application/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.
 
 // This example application illustrates how to use Mojo Tracing from Dart code.
 //
 // To run this app:
 // mojo/devtools/common/mojo_run --trace-startup \
 //   https://core.mojoapps.io/examples/dart/traced_application/lib/main.dart
@@ skipping 17 matching lines @@
   void initialize(List<String> args, String url) {
     // This sets up a connection between this application and the Mojo
     // tracing service.
     _tracing = new TracingHelper(this, "example_traced_application");
     _tracing.traceInstant("initialized", "traced_application");
 
     // Now we schedule some random work just so we have something to trace.
     new Timer.periodic(new Duration(seconds: 1), (t) => function1());
   }
 
+  @override
+  void acceptConnection(String requestorUrl, String resolvedUrl,
+      ApplicationConnection connection) {
+    _tracing.traceInstant("connected", "traced_application");
+  }
+
+
   void function1() {
     var trace = _tracing.beginFunction("function1", "traced_application");
     waitForMilliseconds(100);
     function2(42);
     waitForMilliseconds(100);
     trace.end();
   }
 
   void function2(int x) {
     var trace = _tracing.beginFunction("function2", "traced_application",
@@ skipping 38 matching lines @@
 void waitForMilliseconds(int milliseconds) {
   (new MojoMessagePipe()).endpoints[0]
       .handle
       .wait(MojoHandleSignals.kReadable, milliseconds * 1000);
 }
 
 main(List args) {
   MojoHandle appHandle = new MojoHandle(args[0]);
   new TestUsingTracingApp.fromHandle(appHandle);
 }