Add a changelog entry for `Null`.

CHANGELOG.md

 ## 1.22.0
 
 ### Language
 
   * Breaking change: ['Generalized tear-offs'](https://github.com/gbracha/generalizedTearOffs/blob/master/proposal.md)
     are no longer supported, and will cause errors. We updated the language spec
     and added warnings in 1.21, and are now taking the last step to fully
     de-support them. They were previously supported in the VM only.
 
   * The `assert()` statement has been expanded to support an optional second
@@ skipping 11 matching lines @@
 
     ```
     Unhandled exception:
     'file:///Users/mit/tmp/tool/bin/main.dart': Failed assertion: line 9 pos 10:
     'configFile != null': Tool config missing. Please see https://goo.gl/k8iAi for details.
     #0      _AssertionError._doThrowNew (dart:core-patch/errors_patch.dart:33)
     #1      _AssertionError._throwNew (dart:core-patch/errors_patch.dart:29)
     #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
 
   * Remove support for (long-time deprecated) mixin typedefs.
 
 * Pub
 
   * Avoid using a barback asset server for executables unless they actually use
     transformers. This makes precompilation substantially faster, produces
@@ skipping 1422 matching lines @@
     they will keep the Dart process alive until they time out. This fixes the
     handling of persistent connections. Previously, the client would shut down
     immediately after a request.
 
   * **Breaking change:** `HttpServer` no longer compresses all traffic by
     default. The new `autoCompress` property can be set to `true` to re-enable
     compression.
 
 * `dart:isolate`: `Isolate.spawnUri` added the optional `packageRoot` argument,
   which controls how it resolves `package:` URIs.