Refactor Date implementation in VM.

corelib/src/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.
 
 /**
  * Date is the public interface to a point in time.
+ *
+ * It can represent time values that are at a distance of at most
+ * 8,640,000,000,000,000ms (100,000,000 days) from epoch (1970-01-01 UTC). In
+ * other words: [:value.abs() <= 8640000000000000:].
  */
 interface Date extends Comparable, Hashable default DateImplementation {
   // Weekday constants that are returned by [weekday] method:
   static final int MON = 0;
   static final int TUE = 1;
   static final int WED = 2;
   static final int THU = 3;
   static final int FRI = 4;
   static final int SAT = 5;
   static final int SUN = 6;
@@ skipping 177 matching lines @@
   /**
    * Returns a new [Date] with the [duration] subtracted from this instance.
    */
   Date subtract(Duration duration);
 
   /**
    * Returns a [Duration] with the difference of [:this:] and [other].
    */
   Duration difference(Date other);
 }