Update Stream.listen doc and use ?? in some places.

CHANGELOG.md

+## 1.15.0
+
+### Core library changes
+  * Addad `Uri.queryParametersAll` to handle multiple query parameters with
+    the same name.
+
 ## 1.14.0
 
 ### Core library changes
 * `dart:async`
   * Added `Future.any` static method.
   * Added `Stream.fromFutures` constructor.
 
 * `dart:convert`
   * `Base64Decoder.convert` now takes optional `start` and `end` parameters.
 
 * `dart:core`
   * Added `current` getter to `StackTrace` class.
   * Added `Uri.data` getter for `data:` URIs, and `UriData` class for the
     return type.
-  * Addad `Uri.queryParametersAll` to handle multiple query parameters with
-    the same name.
   * Added `growable` parameter to `List.filled` constructor.
   * Added microsecond support to `DateTime`: `DateTime.microsecond`,
     `DateTime.microsecondsSinceEpoch`, and
     `new DateTime.fromMicrosecondsSinceEpoch`.
 
 * `dart:math`
   * `Random` added a `secure` constructor returning a cryptographically secure
     random generator which reads from the entropy source provided by the
     embedder for every generated random value.
 
@@ skipping 716 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.