Add groupBy to Stream.

CHANGELOG.md

 ## 1.24.0
 
 ### Language
 
 #### Strong Mode
 
 ### Core library changes
 
 * `dart:io`
   * Added `Platform.localeName`, needed for accessing the locale on platforms
     that don't store it in an environment variable.
   * Added `ProcessInfo.currentRss` and `ProcessInfo.maxRss` for inspecting
     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 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.
 
@@ skipping 1853 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.