Dart: Renames dart:mojo_blah -> mojo:blah

sky/examples/dart-mojo.sky

 #!mojo mojo:sky_viewer
 <sky>
 <import src="dart-library.sky" as="library" />
 <script>
 import 'dart:async';
 import '/mojo/public/dart/application.dart';
-import 'dart:mojo_bindings';
-import 'dart:mojo_core';
 import 'dart:typed_data';
 import 'dart:sky.internals' as internals;
+import 'mojo:bindings';
+import 'mojo:core';
 
 import 'package:mojo/services/network/public/interfaces/network_service.mojom.dart';
 import 'package:mojo/services/network/public/interfaces/url_loader.mojom.dart';
 import 'package:mojo/public/interfaces/application/shell.mojom.dart' as shell_mojom;
 
 class WGet extends Application {
   NetworkServiceProxy _networkService;
   UrlLoaderProxy _urlLoaderProxy;
 
   WGet.fromHandle(MojoHandle handle) : super.fromHandle(handle);
@@ skipping 52 matching lines @@
 main() {
   var messagePipe = new MojoMessagePipe();
   var wget = new WGet(messagePipe.endpoints[1]);
   wget.listen();
   var shellProxy = new shell_mojom.ShellProxy.fromHandle(new MojoHandle(internals.takeShellProxyHandle()));
   wget.initializeFromShellProxy(shellProxy, ["mojo:wget", "http://www.google.com"], "mojo:wget");
 }
 
 </script>
 </sky>