Add initializeFromShellProxy to application.dart

This CL adds the ability to initialize the Dart Application class with a shell
proxy (rather than with a MojoHandle). This change lets Sky use the Dart
bindings for Mojo.

This CL was reviewed on the SkyDart branch in
https://codereview.chromium.org/919883002/

R=zra@google.com, zra@chromium.org

Review URL: https://codereview.chromium.org/922083002

examples/dart-mojo.sky

@@ -20,7 +20,7 @@ class WGet extends Application {
   WGet.fromHandle(MojoHandle handle) : super.fromHandle(handle);
   WGet(MojoMessagePipeEndpoint endpoint) : super(endpoint);
 
-  void initialize(List<String> args) {
+  void initialize(List<String> args, String url) {
     run(args);
   }
 
@@ -75,7 +75,7 @@ main() {
   var wget = new WGet(messagePipe.endpoints[1]);
   wget.listen();
   var shellProxy = new shell_mojom.ShellProxy.fromHandle(new MojoHandle(internals.passShellProxyHandle()));
-  wget.initializeFromShellProxy(shellProxy, ["mojo:wget", "http://www.google.com"]);
+  wget.initializeFromShellProxy(shellProxy, ["mojo:wget", "http://www.google.com"], "mojo:wget");
 }
 
 </script>