Add StreamSubscription.asFuture.

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 7da445168addd87af12387a9a5ce5eab2e39f13c..8c2c2e8fd404b6ca28bfaf7da1aead421a3067fa 100644
--- a/sdk/lib/async/stream.dart
+++ b/sdk/lib/async/stream.dart
@@ -832,6 +832,20 @@ abstract class StreamSubscription<T> {
    * Resume after a pause.
    */
   void resume();
+
+  /**
+   * Returns a future that handles the [onDone] and [onError] callbacks.
+   *
+   * This method *overwrites* the existing [onDone] and [onError] callbacks
+   * with new ones that complete the returned future.
+   *
+   * In case of an error the subscription will automatically cancel (even
+   * when it was listening with `unsubscribeOnError` set to `false`).

[Lasse Reichstein Nielsen, 2013/04/15 10:19:15]

"This also makes the subscription cancel on any error".

(Rename unsubscribeOnError to cancelOnError everywhere?)

What happens if you use asFuture, and then set a new onError handler,
overwriting the Future's handler? Will it still cancel on error or not?

[floitsch, 2013/04/15 12:16:37]

If you change the onError handler it won't receive the error and thus not
cancel.
I'm almost tempted to make the listener functions on Subscription private...

+   *
+   * In case of a `done` event the future completes with the given
+   * [futureValue].
+   */
+  Future asFuture([var futureValue]);
 }