Mentioning null-aware in 1.12 changelog

BUG=none
R=johnniwinther@google.com

Review URL: https://codereview.chromium.org//1305273005 .

CHANGELOG.md

@@ -13,6 +13,14 @@
 
 ## 1.12.0
 
+### Language changes
+
+* Null-aware operators
+    * `??`: if null operator. `expr1 ?? expr2` evaluates to `expr1` if not `null`, otherwise `expr2`.
+    * `??=`: null-aware assignment. `v ??= expr` causes `v` to be assigned `expr` only if `v` is `null`.
+    * `x?.p`: null-aware access. `x?.p` evaluates to `x.p` if `x` is not `null`, otherwise evaluates to `null`.
+    * `x?.m()`: null-aware method invocation. `x?.m()` invokes `m` only if `x` is not `null`.
+
 ### Core library changes
 
 * `dart:async`