This code appears to have been wrong forever, as it only
threw in strict mode (presumably predating ES2015 const).

In order to get exactly the right behavior, special
handling of sloppy named function expressions is required.
Rather than polluting PropertyAttributes with another
dummy value, this CL simply adds a bool output argument
to Context::Lookup to indicate that case.

Bug: v8:6677
Change-Id: I34daa5080d291808f10cbaefc91d716f0b22963b
Reviewed-on: https://chromium-review.googlesource.com/602690Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Adam Klein <adamk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47207}