Fix some warnings.

frog/lib/date_implementation.dart

 // Copyright (c) 2011, 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.
 
 // TODO(jmesserly): the native class should be the real JS Date.
 // TODO(jimhug): Making the date value non-lazy might be a good path there.
 class DateImplementation implements Date {
   final int value;
-  final TimeZoneImplementation timeZone;
+  final TimeZone timeZone;
 
   factory DateImplementation(int years,
                              int month,
                              int day,
                              int hours,
                              int minutes,
                              int seconds,
                              int milliseconds) {
     return new DateImplementation.withTimeZone(
         years, month, day,
         hours, minutes, seconds, milliseconds,
         new TimeZoneImplementation.local());
   }
 
   DateImplementation.withTimeZone(int years,
                                   int month,
                                   int day,
                                   int hours,
                                   int minutes,
                                   int seconds,
                                   int milliseconds,
-                                  TimeZoneImplementation timeZone)
+                                  TimeZone timeZone)
       : this.timeZone = timeZone,
         value = _valueFromDecomposed(years, month, day,
                                      hours, minutes, seconds, milliseconds,
                                      timeZone.isUtc) {
     if (value === null) throw new IllegalArgumentException();
     _asJs();
   }
 
   DateImplementation.now()
       : timeZone = new TimeZone.local(),
@@ skipping 239 matching lines @@
     return isUtc == other.isUtc;
   }
 
   String toString() {
     if (isUtc) return "TimeZone (UTC)";
     return "TimeZone (Local)";
   }
 
   final bool isUtc;
 }