Add a flag to the bulk fix protocol to support testing

I think it makes more sense for the server to read the config file than
for `dart fix` to read it and then pass the data to server as part of
the protocol, which it would effectively need to be parsed a second time.

But I don't want config files to be used outside of test mode, so I
added the flag to the protocol. Let me know if you can think of a better
approach.

Change-Id: I1c53bc995272451247e84914f2cd6b75a837ee1e
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/180082
Reviewed-by: Phil Quitslund <pquitslund@google.com>
Commit-Queue: Brian Wilkerson <brianwilkerson@google.com>