Fix datetime test.

tests/corelib/date_time_test.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.
 
 import "package:expect/expect.dart";
 
 // Dart test program for DateTime.
 
 bool get supportsMicroseconds =>
     new DateTime.fromMicrosecondsSinceEpoch(1).microsecondsSinceEpoch == 1;
@@ skipping 19 matching lines @@
 }
 
 void testMillisecondsSinceEpoch() {
   var dt1 = new DateTime.now();
   var millisecondsSinceEpoch = dt1.millisecondsSinceEpoch;
   var dt2 = new DateTime.fromMillisecondsSinceEpoch(millisecondsSinceEpoch);
   Expect.equals(millisecondsSinceEpoch, dt2.millisecondsSinceEpoch);
 }
 
 void testMicrosecondsSinceEpoch() {
+  // We chose a millisecondSinceEpoch that is guaranteed to fit into 53
+  // bits when expressed as microseconds. Furthermore the microsecond part
+  // is equal to 0, which means that an implementation that discards (or
+  // rounds) microseconds should yield correct results for the following
+  // test.
   var dt1 = new DateTime.fromMillisecondsSinceEpoch(1);
   var microsecondsSinceEpoch = dt1.microsecondsSinceEpoch;
   var dt2 = new DateTime.fromMicrosecondsSinceEpoch(microsecondsSinceEpoch);
   Expect.equals(microsecondsSinceEpoch, dt2.microsecondsSinceEpoch);
 
   if (!supportsMicroseconds) return;
   dt1 = new DateTime.now();
-  microsecondsSinceEpoch = dt1.millisecondsSinceEpoch;
+  microsecondsSinceEpoch = dt1.microsecondsSinceEpoch;
   dt2 = new DateTime.fromMicrosecondsSinceEpoch(microsecondsSinceEpoch);
   Expect.equals(microsecondsSinceEpoch, dt2.microsecondsSinceEpoch);
 }
 
 void testFarAwayDates() {
   DateTime dt =
       new DateTime.fromMillisecondsSinceEpoch(1000000000000001, isUtc: true);
   Expect.equals(33658, dt.year);
   Expect.equals(9, dt.month);
   Expect.equals(27, dt.day);
@@ skipping 1278 matching lines @@
   testSubAdd();
   testUnderflowAndOverflow();
   testDateStrings();
   testEquivalentYears();
   testExtremes();
   testFarAwayDates();
   testWeekday();
   testToStrings();
   testIsoString();
 }