Rationalize Dart mojo and sky package structure

sky/tests/services/iframe-embed-vmc.sky

 <sky>
 
 <iframe></iframe>
 
 <script>
 import 'dart:async';
 import 'dart:sky';
 import 'dart:sky.internals' as internals;
 import 'dart:typed_data';
 
-import 'package:mojo/public/dart/application.dart';
-import 'package:mojo/public/dart/bindings.dart';
-import 'package:mojo/public/dart/core.dart';
+import 'package:mojo/application.dart';
+import 'package:mojo/bindings.dart';
+import 'package:mojo/core.dart';
 import 'package:mojom/mojo/shell.mojom.dart' as shell_mojom;
 import 'package:mojom/mojo/input_events.mojom.dart' as input_events;
 import 'package:mojom/mojo/view_manager.mojom.dart' as view_manager;
 
 // This include is a hack which only works with sky_server.  For this to work
 // in the sky_sdk, this echo_service would need to make its own dart package.
 import '/gen/services/js/test/echo_service.mojom.dart' as echo_service_mojom;
 
 class IFrameEmbed extends Application {
   IFrameEmbed.fromHandle(MojoHandle handle) : super.fromHandle(handle);
@@ skipping 25 matching lines @@
 
 main() {
   var messagePipe = new MojoMessagePipe();
   var app = new IFrameEmbed(messagePipe.endpoints[1]);
   var shellProxy = new shell_mojom.ShellProxy.fromHandle(
       new MojoHandle(internals.takeShellProxyHandle()));
   app.initializeFromShellProxy(shellProxy, [], "");
 }
 </script>
 </sky>