asynctypes-685526: Propagate promise from helpers._check
asynctypes-685526: Propagate promise from helpers._check

The unit test function (helpers._check) used promises but didn't pass
the promise on to dependant functions. Now it does.

Signed-off-by: Joe Walker <7a17872fbb32c7d760c9a16ee3076daead11efcf@mozilla.com>

joewalker/gcli,mozilla/gcli,mozilla/gcli,mozilla/gcli,joewalker/gcli