Revert "Return futures on Stream.cancel when possible."

CHANGELOG.md

 ## 1.19.0
 
 ### Language changes
 
 * The language now allows a trailing comma after the last argument of a call and
  the last parameter of a function declaration. This can make long argument or
  parameter lists easier to maintain, as commas can be left as-is when
  reordering lines. For details, see SDK issue
  [26644](https://github.com/dart-lang/sdk/issues/26644).
 
 ### Core library changes
 
 * `dart:io`
   * `Socket.connect` with source-address argument is now non-blocking
     on Mac. Was already non-blocking on all other platforms.
   * Report a better error when a bind fails because of a bad source address.
   * Handle HTTP header `charset` parameter with empty value.
 
-* `dart:async`
-  * More aggressively returns a Future on Stream.cancel operations.
-    Discourages to return `null` from `cancel`.
-  * Fixes a few bugs where the cancel future wasn't passed through
-    transformations.
-
 ### Strong Mode
 
 *   New feature - an option to disable implicit casts
     (SDK issue [26583](https://github.com/dart-lang/sdk/issues/26583)),
     see the [documentation](https://github.com/dart-lang/dev_compiler/blob/master/doc/STATIC_SAFETY.md#disable-implicit-casts)
     for usage instructions and examples.
 
 *   New feature - an option to disable implicit dynamic
     (SDK issue [25573](https://github.com/dart-lang/sdk/issues/25573)),
     see the [documentation](https://github.com/dart-lang/dev_compiler/blob/master/doc/STATIC_SAFETY.md#disable-implicit-dynamic)
@@ skipping 1135 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.