This patch combines embedder.dart and shell.dart into one thing. We should now
handle a bunch of error cases better.

  * embedder.connectToApplication has moved to shell.connectToApplication,
    matching the rest of the mojo universe.
  * embedder.connecttoService has moved to shell.connnectToService (and merged
    with shell.requestService).
  * shell.requestService is now shell.connectToService, matching the rest of
    the mojo universe.
  * serviceRegistry has moved from embedder.serviceRegistry to a top-level
    getter.

Fixes #1803