Make errors from StreamController onCancel calls end up in the returned 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 e612b9cc5ff4dcfdca883c3674193407d74c13f6..a3b266f4f3b37b990dd777fe8fa1a4233bcd9e49 100644
--- a/sdk/lib/async/stream.dart
+++ b/sdk/lib/async/stream.dart
@@ -1174,6 +1174,11 @@ abstract class StreamSubscription<T> {
    * for example a file being read, that should be deleted afterwards.
    * In that case, the file may not be able to be deleted successfully
    * until the returned future has completed.
+   *
+   * The future will be completed with a `null` value.
+   * If the cleanup throws, which it really shouldn't, the returned future
+   * will be completed with that error.
+   *
    * Returns `null` if there is no need to wait.
    */
   Future cancel();