[RegExp] dotAll flag

[brettz9]

As it does not seem that core-js polyfills the RegExp constructor to handle the s flag, I'm wondering whether you might be willing to depend on https://github.com/mathiasbynens/regexpu-core (using its dotAllFlag: true), rewriting the pattern to work compatibly?

Might help with #521 also (the namedGroup: true option).

[zloirock]

I'll think about it. It could be simple enough.