[release] Prepare CHANGELOG for 2.7.1

Change-Id: Id7bd33db6b399dc3c6b2f1861a63d570b30f2fbc
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/133062
Reviewed-by: Michael Thomsen <mit@google.com>

Author: athomas

CHANGELOG.md

@@ -127,6 +127,17 @@ representation soon.
 
 [17207]: https://github.com/dart-lang/sdk/issues/17207
 
+## 2.7.1 - 2020-01-32
+
+This is a patch release that improves dart2js compile-time (issue [40217][]).
+
+[40217]: https://github.com/dart-lang/sdk/issues/40217
+
+**Breaking Change**:
+The Dart SDK for macOS is now only available for x64 (issue [39810][]).
+
+[39810]: https://github.com/dart-lang/sdk/issues/39810
+
 ## 2.7.0 - 2019-12-11
 
 **Extension methods** -- which we shipped in preview in 2.6.0 -- are no longer