Fixed some static type warnings in vm core libraries

runtime/lib/date.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.
 // Dart core library.
 
 class TimeZoneImplementation implements TimeZone {
   const TimeZoneImplementation.utc() : isUtc = true;
   TimeZoneImplementation.local() : isUtc = false {}
 
   bool operator ==(Object other) {
@@ skipping 19 matching lines @@
         new TimeZoneImplementation.local());
   }
 
   DateImplementation.withTimeZone(int years,
                                   int month,
                                   int day,
                                   int hours,
                                   int minutes,
                                   int seconds,
                                   int milliseconds,
-                                  TimeZoneImplementation timeZone)
+                                  TimeZone timeZone)
   : timeZone = timeZone,
     value = _brokenDownDateToMillisecondsSinceEpoch(
                years, month, day, hours, minutes, seconds, milliseconds,
                timeZone.isUtc) {
     if (value === null) throw new IllegalArgumentException();
   }
 
   DateImplementation.now()
   : timeZone = new TimeZone.local(),
     value = getCurrentMs_() {
@@ skipping 366 matching lines @@
 
   static int getHours_(int secondsSinceEpoch, bool isUtc)
       native "DateNatives_getHours";
 
   static int getMinutes_(int secondsSinceEpoch, bool isUtc)
       native "DateNatives_getMinutes";
 
   static int getSeconds_(int secondsSinceEpoch, bool isUtc)
       native "DateNatives_getSeconds";
 }