Make `String#contains(regex)` throw

This was a spec bug that is finally fixed in rev23 of the ES6 draft as per <https://bugs.ecmascript.org/show_bug.cgi?id=2407>.

I’ll merge this as soon as the new draft gets published so I can confirm whether this is the correct order of operations. (I assume it is, since this matches `String#{starts,ends}With`.)