Update changelog for DateTime.microsecond support.

CHANGELOG.md

Index: CHANGELOG.md
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 087ba199b299eed344aeac6615b9e3ef13c5aed4..adfe20b5784eddf887d64bbf4e0c2cd60546f71c 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -9,6 +9,9 @@
   * Added `Uri.data` getter for `data:` URIs, and `UriData` class for the
     return type.
   * Added `growable` parameter to `List.filled` constructor.
+  * Added microsecond support to `DateTime`: `DateTime.microsecond`,
+    `DateTime.microsecondsSinceEpoch`, and
+    `new DateTime.fromMicrosecondsSinceEpoch`.
 
 * `dart:math`
   * `Random` added a `secure` constructor returning a cryptographically secure