This is an overly pessimistic approach where PreParser only keeps
track of unresolved variables, but doesn't declare anything. This
will result in context-allocating variables in the outer function
unnecessarily, if the variable names clash with variable names
used by the inner function (even if the variables are not the
same). However, we have been unable to prove that this approach
wouldn't be good enough for the practical purposes.

Review-Url: https://codereview.chromium.org/2322243002
Cr-Commit-Position: refs/heads/master@{#39469}