[*kit] fulfil promise immediately

I think this would only commonly happen for cases which files aren't
being loaded (debug asmjs), but could happen if multiple ready() calls
are made (which is discouraged).

Bug: skia:
Change-Id: If848aad8603f997661502563136ac590cc9a6d5a
Reviewed-on: https://skia-review.googlesource.com/c/181409
Reviewed-by: Kevin Lubick <kjlubick@google.com>