Let StreamSubscription.cancel return a Future.

sdk/lib/async/stream.dart

Index: sdk/lib/async/stream.dart
diff --git a/sdk/lib/async/stream.dart b/sdk/lib/async/stream.dart
index ab31d16a5dbf6837bf9d648fa6500a7a0ef1bfde..6672f475a9373b631318cea00d8d35dc2cd5c100 100644
--- a/sdk/lib/async/stream.dart
+++ b/sdk/lib/async/stream.dart
@@ -336,7 +336,7 @@ abstract class Stream<T> {
         if (seenFirst) {
           _runUserCode(() => combine(value, element),
                        (T newValue) { value = newValue; },
-                       _cancelAndError(subscription, result));
+                       _cancelAndErrorClosure(subscription, result));
         } else {
           value = element;
           seenFirst = true;
@@ -365,7 +365,7 @@ abstract class Stream<T> {
         _runUserCode(
           () => combine(value, element),
           (newValue) { value = newValue; },
-          _cancelAndError(subscription, result)
+          _cancelAndErrorClosure(subscription, result)
         );
       },
       onError: (e, st) {
@@ -402,8 +402,7 @@ abstract class Stream<T> {
         try {
           buffer.write(element);
         } catch (e, s) {
-          subscription.cancel();
-          result._completeError(_asyncError(e, s));
+          _cancelAndError(subscription, result, _asyncError(e, s), s);
         }
       },
       onError: (e) {
@@ -431,11 +430,10 @@ abstract class Stream<T> {
             () => (element == needle),
             (bool isMatch) {
               if (isMatch) {
-                subscription.cancel();
-                future._complete(true);
+                _cancelAndValue(subscription, future, true);
               }
             },
-            _cancelAndError(subscription, future)
+            _cancelAndErrorClosure(subscription, future)
           );
         },
         onError: future._completeError,
@@ -461,7 +459,7 @@ abstract class Stream<T> {
           _runUserCode(
             () => action(element),
             (_) {},
-            _cancelAndError(subscription, future)
+            _cancelAndErrorClosure(subscription, future)
           );
         },
         onError: future._completeError,
@@ -487,11 +485,10 @@ abstract class Stream<T> {
             () => test(element),
             (bool isMatch) {
               if (!isMatch) {
-                subscription.cancel();
-                future._complete(false);
+                _cancelAndValue(subscription, future, false);
               }
             },
-            _cancelAndError(subscription, future)
+            _cancelAndErrorClosure(subscription, future)
           );
         },
         onError: future._completeError,
@@ -517,11 +514,10 @@ abstract class Stream<T> {
             () => test(element),
             (bool isMatch) {
               if (isMatch) {
-                subscription.cancel();
-                future._complete(true);
+                _cancelAndValue(subscription, future, true);
               }
             },
-            _cancelAndError(subscription, future)
+            _cancelAndErrorClosure(subscription, future)
           );
         },
         onError: future._completeError,
@@ -553,8 +549,7 @@ abstract class Stream<T> {
     StreamSubscription subscription;
     subscription = this.listen(
       (_) {
-        subscription.cancel();
-        future._complete(false);
+        _cancelAndValue(subscription, future, false);
       },
       onError: future._completeError,
       onDone: () {
@@ -686,9 +681,7 @@ abstract class Stream<T> {
     StreamSubscription subscription;
     subscription = this.listen(
       (T value) {
-        subscription.cancel();
-        future._complete(value);
-        return;
+        _cancelAndValue(subscription, future, value);
       },
       onError: future._completeError,
       onDone: () {
@@ -742,10 +735,9 @@ abstract class Stream<T> {
     subscription = this.listen(
       (T value) {
         if (foundResult) {
-          subscription.cancel();
           // This is the second element we get.
           Error error = new StateError("More than one element");
-          future._completeError(error);
+          _cancelAndError(subscription, future, error, null);
           return;
         }
         foundResult = true;
@@ -786,11 +778,10 @@ abstract class Stream<T> {
           () => test(value),
           (bool isMatch) {
             if (isMatch) {
-              subscription.cancel();
-              future._complete(value);
+              _cancelAndValue(subscription, future, value);
             }
           },
-          _cancelAndError(subscription, future)
+          _cancelAndErrorClosure(subscription, future)
         );
       },
       onError: future._completeError,
@@ -827,7 +818,7 @@ abstract class Stream<T> {
               result = value;
             }
           },
-          _cancelAndError(subscription, future)
+          _cancelAndErrorClosure(subscription, future)
         );
       },
       onError: future._completeError,
@@ -864,16 +855,18 @@ abstract class Stream<T> {
           (bool isMatch) {
             if (isMatch) {
               if (foundResult) {
-                subscription.cancel();
-                future._completeError(
-                    new StateError('Multiple matches for "single"'));
+                _cancelAndError(
+                    subscription,
+                    future,
+                    new StateError('Multiple matches for "single"'),
+                    null);
                 return;
               }
               foundResult = true;
               result = value;
             }
           },
-          _cancelAndError(subscription, future)
+          _cancelAndErrorClosure(subscription, future)
         );
       },
       onError: future._completeError,
@@ -906,8 +899,7 @@ abstract class Stream<T> {
     subscription = this.listen(
       (T value) {
         if (index == 0) {
-          subscription.cancel();
-          future._complete(value);
+          _cancelAndValue(subscription, future, value);
           return;
         }
         index -= 1;
@@ -935,8 +927,11 @@ abstract class StreamSubscription<T> {
    *
    * If an event is currently firing, this unsubscription will only
    * take effect after all subscribers have received the current event.
+   *
+   * Returns a future if the cancel-operation is not completed synchronously.
+   * Otherwise returns `null`.
    */
-  void cancel();
+  Future cancel();
 
   /** Set or override the data event handler of this subscription. */
   void onData(void handleData(T data));
@@ -1220,6 +1215,9 @@ abstract class StreamIterator<T> {
    * If you need to stop listening for values before the stream iterator is
    * automatically closed, you must call [cancel] to ensure that the stream
    * is properly closed.
+   *
+   * Returns a future if the cancel-operation is not completed synchronously.
+   * Otherwise returns `null`.
    */
-  void cancel();
+  Future cancel();
 }