The dotAll flag changes behavior of the dot '.' character to match every
possible single character instead of excluding certain line terminators.

The implementation is staged behind --harmony-regexp-dotall.

Spec proposal: https://github.com/mathiasbynens/es-regexp-dotall-flag

BUG=v8:6172

Review-Url: https://codereview.chromium.org/2780173002
Cr-Commit-Position: refs/heads/master@{#44295}