[es6] stage sticky regexps and RegExp.prototype.flags (issue 4342).

Performance and stability improvements on all platforms.

Cr-Commit-Position: refs/heads/4.8.111@{#1}
Cr-Branched-From: 89a07ba4a0fcb7c1557c29aceffcabbcf61041a6-refs/heads/master@{#31395}