Object::GetProperty fails if the given name is a valid array index.
This CL switches to Object::GetPropertyOrElement for lookups of imports.
The new tests check that we now accept numbers as module name or
function name in FFI.

R=ahaas@chromium.org, titzer@chromium.org

Review-Url: https://codereview.chromium.org/2503313002
Cr-Commit-Position: refs/heads/master@{#41022}