Dart: Reorganize files

mojo/dart/unittests/http_load_test/lib/main.dart

 #!mojo mojo:dart_content_handler
 
 library test;
 
 import 'dart:async';
 import 'dart:mojo.internal';
 
 import 'package:mojo_services/tracing/tracing.mojom.dart' as tracing;
 
 part 'src/part0.dart';
 
 main(List args) {
   // Hang around for a sec and then close the shell handle.
   new Timer(new Duration(milliseconds: 1000), () {
     print('PASS');
     MojoHandleNatives.close(args[0]);
   });
 }