Sky should expose services provided to/by embedder

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;
@@ skipping 56 matching lines @@
     _networkService.close();
     _urlLoaderProxy = null;
     _networkService = null;
   }
 }
 
 main() {
   var messagePipe = new MojoMessagePipe();
   var wget = new WGet(messagePipe.endpoints[1]);
   wget.listen();
-  var shellProxy = new shell_mojom.ShellProxy.fromHandle(new MojoHandle(internals.passShellProxyHandle()));
+  var shellProxy = new shell_mojom.ShellProxy.fromHandle(new MojoHandle(internals.takeShellProxyHandle()));
   wget.initializeFromShellProxy(shellProxy, ["mojo:wget", "http://www.google.com"], "mojo:wget");
 }
 
 </script>
 </sky>