In one of my previous refactoring changes, I lost the fact that the
context should be detached before invalidating a file's contents. If
this function is canceled, we will be in a bad state.
Also, small change to return ctx.Err() instead of a custom error message
from (*packageHandle).check.
Change-Id: I19e513e09e438feee105fdd89cb7364a0c3c5e7f
Reviewed-on: https://go-review.googlesource.com/c/tools/+/212104
Run-TryBot: Rebecca Stambler <rstambler@golang.org>
Reviewed-by: Heschi Kreinick <heschi@google.com>