asynctypes-685526: Review fix: s/withTab/addTab

Review comments indicated that 'addTab' was a much easier-to-understand
name.

Signed-off-by: Joe Walker <jwalker@mozilla.com>
asynctypes-685526: fix sync promise breakage

If we had a promise on an attribute that turned out to be already
resolved then previously we cleared the value after we set it. Now we
don't.

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