Dart: Renames dart:mojo_blah -> mojo:blah

examples/dart/hello_world/hello/main.dart

 // 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.
 
 // To run this app:
 // mojo_shell mojo:hello
 
-import 'dart:mojo_application';
-import 'dart:mojo_bindings';
-import 'dart:mojo_core';
+import 'mojo:application';
+import 'mojo:bindings';
+import 'mojo:core';
 
 class Hello extends Application {
   Hello.fromHandle(MojoHandle handle) : super.fromHandle(handle);
 
   void initialize(List<String> args, String url) {
     print("$url Hello");
     // We expect to find the world mojo application at the same
     // path as this application.
     connectToApplication(url.replaceAll("hello", "world"));
     close();
   }
 }
 
 main(List args) {
   MojoHandle appHandle = new MojoHandle(args[0]);
   var hello = new Hello.fromHandle(appHandle);
   hello.listen();
 }