Sloppy-mode let parsing

Sloppy-mode let parsing

This patch makes 'let' a contextual keyword in both strict and sloppy mode.
It behaves as a keyword when used at the beginning of a StatementListItem
or lexical declaration at the beginning of a for statement, if it is followed
by an identifier, [ or {. Implementing this change requires an extra token
look-ahead by the parser which is only invoked in certain cases (so as to
avoid parsing RegExps as ECMAScript tokens). This might result in a slowdown
of the scanner, but performance testing of this patch hasn't yet found much
of a regression.

BUG=v8:3305
LOG=Y
R=adamk,vogelheim

Review URL: https://codereview.chromium.org/1315673009

Cr-Commit-Position: refs/heads/master@{#30451}