The old handling of escaped keywords erroneously treated escaped versions
of "let" and "static" as ESCAPED_KEYWORD, leading to erroneous errors in
sloppy mode. Moreover, though the class literal parsing code attempted
to fix up the parsing of escaped versions of "static" to allow it in the
right places, that code wasn't complete.

Fixing the scanner to mark escaped "static" as ESCAPED_STRICT_RESERVED_WORD
allows simplifying the class literal parsing code. A little extra code
was needed to properly handle the new treatment of escaped "let".

Note that "yield" is still broken (that is, we're overly restrictive of
escaped "yield" in sloppy mode).

Review URL: https://codereview.chromium.org/1602013007

Cr-Commit-Position: refs/heads/master@{#33396}