Generators were previously treated as "top level" for preparsing purposes,
since all their variables are context-allocated. But doing so isn't quite
correct: the allocation of the "arguments" variable for a generator
depends on whether it's referenced, and so an inner arrow function
which references "arguments" won't properly trigger allocation of
"arguments" since the reference will not be noticed in the preparser.

The same problem exists for "this" since commit 68f0a47b;
before that commit, all generators implicitly referenced their "this" argument
as part of the desugaring. With that implicit reference gone, "this"
falls into the same problem as arguments.

This patch restricts the special "top level" treatment to modules,
which have only a trivial "this" binding (it's always undefined), and no
arguments binding. Moreover, all code inside modules is strict, meaning
that unresolved references to "this" will also result in undefined.

R=marja@chromium.org

Bug: chromium:723132
Change-Id: I814d145fb8f3f1a65abb48e4e35595428d063051
Reviewed-on: https://chromium-review.googlesource.com/508055Reviewed-by: Marja Hölttä <marja@chromium.org>
Commit-Queue: Adam Klein <adamk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45399}