The language mode is no longer constant accross a compilation unit. For
example the extends clause of a class literal can be in strict mode even
though the surrounding function is in sloppy mode. This makes any global
language mode predicate that reasons over an entire function inherently
dangerous. Instead one should use the appropriate predicate on scopes or
literals directly.

R=bmeurer@chromium.org

Review-Url: https://codereview.chromium.org/1949013002
Cr-Commit-Position: refs/heads/master@{#36010}