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

dart/frog/leg/lib/mockimpl.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.
 
 // Mocks of classes and interfaces that Leg cannot read directly.
 
 // TODO(ahe): Remove this file.
 
 class JSSyntaxRegExp implements RegExp {
   final String pattern;
@@ skipping 60 matching lines @@
       List<String> this._groups);
 
   final String pattern;
   final String str;
   final int _start;
   final int _end;
   final List<String> _groups;
 
   int start() => _start;
   int end() => _end;
-  String group(int index) => stringify(_groups[index]);
+  String group(int index) => _groups[index];
   String operator [](int index) => group(index);
   int groupCount() => _groups.length - 1;
 
   List<String> groups(List<int> groups) {
     List<String> out = [];
     for (int i in groups) {
       out.add(group(i));
     }
     return out;
   }
@@ skipping 357 matching lines @@
     }
     String twoDigitMinutes =
         twoDigits(inMinutes.remainder(Duration.MINUTES_PER_HOUR));
     String twoDigitSeconds =
         twoDigits(inSeconds.remainder(Duration.SECONDS_PER_MINUTE));
     String threeDigitMs =
         threeDigits(inMilliseconds.remainder(Duration.MILLISECONDS_PER_SECOND));
     return "${inHours}:${twoDigitMinutes}:${twoDigitSeconds}.${threeDigitMs}";
   }
 }