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

CHANGELOG.md

Index: CHANGELOG.md
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 47fd5c18ffb87d0bc6f64a4f7fb26efaab8641ba..277a983fb200a663d5481e28429793d8814213f4 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,3 +1,9 @@
+## 1.15.0
+
+### Core library changes
+  * Addad `Uri.queryParametersAll` to handle multiple query parameters with
+    the same name.
+
 ## 1.14.0
 
 ### Core library changes
@@ -12,8 +18,6 @@
   * 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