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

This reverts commit 1905dda.

Review URL: https://codereview.chromium.org/2223133002 .

CHANGELOG.md

@@ -16,12 +16,6 @@
   * 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/lib/async/stream.dart

@@ -232,7 +232,6 @@ abstract class Stream<T> {
         onCancel: () {
           if (timer != null) timer.cancel();
           timer = null;
-          return Future._nullFuture;
         });
     return controller.stream;
   }
@@ -442,7 +441,7 @@ abstract class Stream<T> {
         onListen: onListen,
         onPause: () { subscription.pause(); },
         onResume: () { subscription.resume(); },
-        onCancel: () => subscription.cancel(),
+        onCancel: () { subscription.cancel(); },
         sync: true
       );
     }
@@ -500,7 +499,7 @@ abstract class Stream<T> {
         onListen: onListen,
         onPause: () { subscription.pause(); },
         onResume: () { subscription.resume(); },
-        onCancel: () => subscription.cancel(),
+        onCancel: () { subscription.cancel(); },
         sync: true
       );
     }
@@ -1408,10 +1407,7 @@ abstract class StreamSubscription<T> {
    * the subscription is canceled.
    *
    * Returns a future that is completed once the stream has finished
-   * its cleanup.
-   *
-   * For historical reasons, may also return `null` if no cleanup was necessary.
-   * Returning `null` is deprecated and should be avoided.
+   * its cleanup. May also return `null` if no cleanup was necessary.
    *
    * Typically, futures are returned when the stream needs to release resources.
    * For example, a stream might need to close an open file (as an asynchronous
@@ -1715,7 +1711,7 @@ abstract class StreamTransformer<S, T> {
    *             },
    *             onPause: () { subscription.pause(); },
    *             onResume: () { subscription.resume(); },
-   *             onCancel: () => subscription.cancel(),
+   *             onCancel: () { subscription.cancel(); },
    *             sync: true);
    *           return controller.stream.listen(null);
    *         });

sdk/lib/async/stream_impl.dart

@@ -188,10 +188,9 @@ class _BufferingStreamSubscription<T> implements StreamSubscription<T>,
     // error or done event pending (waiting for the cancel to be done) discard
     // that event.
     _state &= ~_STATE_WAIT_FOR_CANCEL;
-    if (!_isCanceled) {
-      _cancel();
-    }
-    return _cancelFuture ?? Future._nullFuture;
+    if (_isCanceled) return _cancelFuture;
+    _cancel();
+    return _cancelFuture;
   }
 
   Future/*<E>*/ asFuture/*<E>*/([var/*=E*/ futureValue]) {
@@ -200,14 +199,8 @@ class _BufferingStreamSubscription<T> implements StreamSubscription<T>,
     // Overwrite the onDone and onError handlers.
     _onDone = () { result._complete(futureValue); };
     _onError = (error, stackTrace) {
-      Future cancelFuture = cancel();
-      if (!identical(cancelFuture, Future._nullFuture)) {
-        cancelFuture.whenComplete(() {
-          result._completeError(error, stackTrace);
-        });
-      } else {
-        result._completeError(error, stackTrace);
-      }
+      cancel();
+      result._completeError(error, stackTrace);
     };
 
     return result;
@@ -368,8 +361,7 @@ class _BufferingStreamSubscription<T> implements StreamSubscription<T>,
     if (_cancelOnError) {
       _state |= _STATE_WAIT_FOR_CANCEL;
       _cancel();
-      if (_cancelFuture is Future &&
-          !identical(_cancelFuture, Future._nullFuture)) {
+      if (_cancelFuture is Future) {
         _cancelFuture.whenComplete(sendError);
       } else {
         sendError();
@@ -397,8 +389,7 @@ class _BufferingStreamSubscription<T> implements StreamSubscription<T>,
 
     _cancel();
     _state |= _STATE_WAIT_FOR_CANCEL;
-    if (_cancelFuture is Future &&
-        !identical(_cancelFuture, Future._nullFuture)) {
+    if (_cancelFuture is Future) {
       _cancelFuture.whenComplete(sendDone);
     } else {
       sendDone();
@@ -787,7 +778,7 @@ class _DoneStreamSubscription<T> implements StreamSubscription<T> {
     }
   }
 
-  Future cancel() => Future._nullFuture;
+  Future cancel() => null;
 
   Future/*<E>*/ asFuture/*<E>*/([var/*=E*/ futureValue]) {
     _Future/*<E>*/ result = new _Future/*<E>*/();
@@ -925,7 +916,7 @@ class _BroadcastSubscriptionWrapper<T> implements StreamSubscription<T> {
 
   Future cancel() {
     _stream._cancelSubscription();
-    return Future._nullFuture;
+    return null;
   }
 
   bool get isPaused {
@@ -1041,7 +1032,7 @@ class _StreamIteratorImpl<T> implements StreamIterator<T> {
 
   Future cancel() {
     StreamSubscription subscription = _subscription;
-    if (subscription == null) return Future._nullFuture;
+    if (subscription == null) return null;
     if (_state == _STATE_MOVING) {
       _Future<bool> hasNext = _futureOrPrefetch as Object /*=_Future<bool>*/;
       _clear();

sdk/lib/async/stream_transformers.dart

@@ -109,7 +109,7 @@ class _SinkTransformerStreamSubscription<S, T>
     if (_isSubscribed) {
       StreamSubscription subscription = _subscription;
       _subscription = null;
-      return subscription.cancel();
+      subscription.cancel();
     }
     return null;
   }