Mentioning null-aware in 1.12 changelog

CHANGELOG.md

 ## 1.13.0
 
 * `dart:core`
   * `Uri` added `removeFragment` method.
   * `String.allMatches` (implementing `Pattern.allMatches`) is now lazy,
     as all `allMatches` implementations are intended to be.
 * `dart:io`
   * `HttpClient` no longer sends URI fragments in the requeust. This is not
     allowed by the HTTP protocol.
     The `HttpServer` still gracefully receives fragments, but discards them
     before delivering the request.
 * `dart:async`
   * `StreamController` added getters for the `onListen`, `onPause`, `onResume`
     and `onCancel` callbacks.
 
 
 ## 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`.

[eernst, 2015/08/25 12:28:36]

You could say that the reader of this announcement could as well go to the spec
and read about the details, so maybe there is no need to have this summary here.

Otherwise, it might be nice to make it explicitly inofficial: "

```
* Null-aware operators implemented. Informal summary of the semantics:
    * `??`: ...
```

+
 ### Core library changes
 
 * `dart:async`
   * `StreamController` added setters for the `onListen`, `onPause`, `onResume`
     and `onCancel` callbacks.
 
 * `dart:convert`
   * `LineSplitter` added a `split` static method returning an `Iterable`.
 
 * `dart:core`
@@ skipping 482 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.