Add a changelog entry for `Null`.

CHANGELOG.md

Index: CHANGELOG.md
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 04f687745d4a0e8638fc425d18e53b0206b2dda1..32e757d90318db27bb6570ad4434b33a821e7f35 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -29,6 +29,20 @@
     #2      main (file:///Users/mit/tmp/tool/bin/main.dart:9:10)
     ```
 
+  * The `Null` type has been moved to the bottom of the type hierarchy. As such,
+    it is considered a subtype of every other type.
+
+    Examples:
+    ```
+    Null foo() => null;
+    int x = foo();
+    String x = foo();
+
+    List<Null> bar() => <Null>[];
+    List<int> = bar();
+    List<String> = bar();
+    ```
+
 ### Tool changes
 
 * Dart2Js