Warn when adding something to a closed sink and improve documentation

CHANGELOG.md

Index: CHANGELOG.md
diff --git a/CHANGELOG.md b/CHANGELOG.md
index ca9c9438e0313f2e116baf9192d94633d6ea31db..4cc71eb413d2ae1e71ba485429c3377b1c40cd84 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -17,6 +17,11 @@
   * JSON maps are now typed as `Map<String, dynamic>` instead of
     `Map<dynamic, dynamic>`. A JSON-map is not a `HashMap` or `LinkedHashMap`
     anymore (but just a `Map`).
+* `dart:async`, `dart:io`, `dart:core`
+    * Adding to a closed sink, including `IOSink`, is not allowed anymore. In
+      1.24, violations are only reported (on stdout or stderr), but a future
+      version of the Dart SDK will change this to throwing a `StateError`.
+
 
 ### Dart VM