Move the --harmony-sloppy-let flag to staging for further testing, and
update test262 for the new passing tests. Also increase the strictness
of the parser, even in sloppy mode, to disallow "new legacy compat" for

  for (let x = 5 in {}) {}

which is now a SyntaxError.

BUG=v8:3305
LOG=Y
R=adamk

Review URL: https://codereview.chromium.org/1321013005

Cr-Commit-Position: refs/heads/master@{#30525}