Add support for 'u' and 'y' RegExp flags in ES6 mode.

These flags are defined in section 12.2.7.1 of the draft spec: http://people.mozilla.org/\~jorendorff/es6-draft.html\#sec-primary-expression-regular-expression-literals-static-semantics-early-errors (narrowing the RegularExpressionFlags production defined in ẞ11.8.5).  The new flags are described at https://mathiasbynens.be/notes/es6-unicode-regex.