ES#sec-regexp.prototype.compile RegExp.prototype.compile (pattern, flags)
function RegExpCompileJS(pattern, flags) {
  if (!IS_REGEXP(this)) {
    throw MakeTypeError(kIncompatibleMethodReceiver,
                        "RegExp.prototype.compile", this);
  }

  if (IS_REGEXP(pattern)) {
    if (!IS_UNDEFINED(flags)) throw MakeTypeError(kRegExpFlags);

    flags = PatternFlags(pattern);
    pattern = REGEXP_SOURCE(pattern);
  }

  RegExpInitialize(this, pattern, flags);

  // Return undefined for compatibility with JSC.
  // See http://crbug.com/585775 for web compat details.
}