Don't stringify unmatched groups and throw IllegalArgumentException for bad date strings.

dart/frog/leg/lib/regexp_helper.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 class RegExpWrapper {
   final re;
 
   // TODO(ahe): This constructor is clearly not const. We need some
   // better way to handle constant regular expressions. One might
   // question if regular expressions are really constant as we have
@@ skipping 22 matching lines @@
   static makeRegExp(pattern, flags) {
     checkString(pattern);
     try {
       return JS('Object', @'new RegExp($0, $1)', pattern, flags);
     } catch (var e) {
       throw new IllegalJSRegExpException(pattern,
                                          JS('String', @'String($0)', e));
     }
   }
 }
-
-stringify(x) => x === null ? "" : x.toString();