Dart: Renames dart:mojo_blah -> mojo:blah

mojo/dart/test/handle_watcher_test.dart

 // Copyright 2014 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:isolate';
-import 'dart:mojo_core';
 import 'dart:typed_data';
+import 'mojo:core';
 
 import 'package:mojo/dart/testing/expect.dart';
 
 void simpleTest() {
   var pipe = new MojoMessagePipe();
   Expect.isNotNull(pipe);
 
   var endpoint = pipe.endpoints[0];
   Expect.isTrue(endpoint.handle.isValid);
 
@@ skipping 121 matching lines @@
   Expect.equals(10, pings);
   Expect.equals(10, pongs);
 }
 
 
 main() async {
   simpleTest();
   await simpleAsyncAwaitTest();
   await pingPongTest();
 }