simplify mozilla AST RegExpLiteral token parse and handle corner cases of regex.pattern better

Author: kzc

lib/mozilla-ast.js

@@ -149,10 +149,7 @@
                 var rx = M.regex;
                 if (rx && rx.pattern) {
                     // RegExpLiteral as per ESTree AST spec
-                    args.value = "/" + rx.pattern + "/";
-                    if (rx.flags) {
-                        args.value += rx.flags;
-                    }
+                    args.value = new RegExp(rx.pattern, rx.flags).toString();
                 } else {
                     // support legacy RegExp
                     args.value = M.regex && M.raw ? M.raw : val;