Dart: Renames dart:mojo_blah -> mojo:blah

This improves consistency with the convention that the dart: scheme is
only for things from the Dart standard library.

BUG=
R=erg@chromium.org

Review URL: https://codereview.chromium.org/951783004

examples/dart-mojo.sky

@@ -4,10 +4,10 @@
 <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';