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
fab160cfce