Warn when adding something to a closed sink and improve documentation

CHANGELOG.md

 ## 1.24.0
 
 ### Language
 * During a dynamic type check, `void` is not required to be `null` anymore.
   In practice, this makes overriding `void` functions with non-`void` functions
   safer.
 
 #### Strong Mode
 
 * Removed ad hoc Future.then inference in favor of using FutureOr.  Prior to
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     the Dart VM process current and peak resident set size.
   * Added 'RawSynchronousSocket', a basic synchronous socket implementation.
 * `dart:convert`
   * Removed deprecated `ChunkedConverter` class.
   * 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`
   * Add `groupBy` to `Stream`. Allows splitting a string into separate streams
     depending on "key" property computed from the individual events.
+* `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
 
 ### Tool Changes
 
 * Pub
     * `pub build` will use a failing exit code if there are errors in any
       transformer.
     * Allow publishing packages that depend on the Flutter SDK.
 
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     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.