Dart: Renames dart:mojo_blah -> mojo:blah

mojo/dart/test/core_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:mojo_core';
 import 'dart:typed_data';
+import 'mojo:core';
 
 import 'package:mojo/dart/testing/expect.dart';
 
 invalidHandleTest() {
   MojoHandle invalidHandle = new MojoHandle(MojoHandle.INVALID);
 
   // Close.
   MojoResult result = invalidHandle.close();
   Expect.isTrue(result.isInvalidArgument);
 
@@ skipping 280 matching lines @@
   duplicate = null;
 }
 
 
 main() {
   invalidHandleTest();
   basicMessagePipeTest();
   basicDataPipeTest();
   basicSharedBufferTest();
 }