This was being allowed due to the use of BindingFlags instead of VariableMode
to determine whether a looked-up binding was lexical. Because function
declarations are hoisted, they never need hole checks, and so were being
miscategorized as non-lexical.

This patch augments Context::Lookup with a VariableMode out param, which
allows this check to determine precisely whether the binding is lexical.

BUG=v8:4454, v8:5256

Review-Url: https://codereview.chromium.org/2206483004
Cr-Commit-Position: refs/heads/master@{#38260}