Implement Duration.

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 {
   JSSyntaxRegExp(String pattern,
@@ skipping 25 matching lines @@
     if (!isJSArray(charCodes)) {
       if (charCodes is !List) throw new IllegalArgumentException(charCodes);
       charCodes = new List.from(charCodes);
     }
     return Primitives.stringFromCharCodes(charCodes);
   }
 
   static String join(List<String> strings, String separator) {
     checkNull(strings);
     checkNull(separator);
-    if (separator is !String) throw new IllegalArgumentException(separator);
     var result = "";
     var first = true;
     for (var string in strings) {
       checkNull(string);
       if (string is !String) throw new IllegalArgumentException(string);
       if (!first) result += separator; // TODO(ahe): Use string buffer.
       result += string; // TODO(ahe): Use string buffer.
       first = false;
     }
     return result;
@@ skipping 216 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}";
   }
 }