ES6 computed property names

This adds support for computed property names, under the flag
--harmony-computed-property-names, for both object literals and
classes.

This is a revert of the revert, a76419f0f497db156eb1010e9b4be398951280cb with
no changes. I cannot reproduce the issue on Win8.1 or WinXP. Letting the bots
try again.

BUG=v8:3754
LOG=Y
TBR=dslomov@chromium.org

Review URL: https://codereview.chromium.org/807173002

Cr-Commit-Position: refs/heads/master@{#25851}