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