Fix datetime test.

tests/corelib/date_time_test.dart

Index: tests/corelib/date_time_test.dart
diff --git a/tests/corelib/date_time_test.dart b/tests/corelib/date_time_test.dart
index 7c2940a4c5723f8e6c12366fae090710abe418d7..9a9de1cfee66dbf6122321c8030f8f8f741d4c25 100644
--- a/tests/corelib/date_time_test.dart
+++ b/tests/corelib/date_time_test.dart
@@ -37,6 +37,11 @@ void testMillisecondsSinceEpoch() {
 }
 
 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);
@@ -44,7 +49,7 @@ void testMicrosecondsSinceEpoch() {
 
   if (!supportsMicroseconds) return;
   dt1 = new DateTime.now();
-  microsecondsSinceEpoch = dt1.millisecondsSinceEpoch;
+  microsecondsSinceEpoch = dt1.microsecondsSinceEpoch;
   dt2 = new DateTime.fromMicrosecondsSinceEpoch(microsecondsSinceEpoch);
   Expect.equals(microsecondsSinceEpoch, dt2.microsecondsSinceEpoch);
 }