Re-remove ad hoc Future.then inference.

CHANGELOG.md

 ## 1.24.0
 
 ### Language
 
 #### Strong Mode
 
+* Removed ad hoc Future.then inference in favor of using FutureOr.  Prior to
+adding FutureOr to the language, the analyzer implented an ad hoc type inference
+for Future.then (and overrides) treating it as if the onValue callback was typed
+to return FutureOr for the purposes of inference.  This ad hoc inference has
+been removed now that FutureOr has been added.
+
+Packages that implement `Future` must either type the `onValue` parameter to
+`.then` as returning `FutureOr<T>`, or else must leave the type of the parameter
+entirely to allow inference to fill in the type.
+
+
 ### 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.
@@ skipping 1866 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.