Let StreamSubscription.cancel return a Future.

tests/lib/async/stream_state_nonzero_timer_test.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 // Test the event/callback protocol of the stream implementations.
 // Uses a non-zero timer so it fails on d8.
 
 library stream_state_nonzero_timer_test;
 
 import "dart:async";
@@ skipping 12 matching lines @@
   test("$p-sub-data/pause/resume/pause/resume-done", () {
     var t = new StreamProtocolTest();
     t..expectListen()
      ..expectData(42, () {
          t.pause();
        })
      ..expectPause(() { t.resume(); })
      ..expectResume(() { t.pause(); })
      ..expectPause(() { t.resume(); })
      ..expectResume(() { t.close(); })
-     ..expectDone()
-     ..expectCancel(t.terminate);
+     ..expectCancel()
+     ..expectDone(t.terminate);
     t..listen()..add(42);
   });
 
   test("$p-sub-data/pause-done", () {
     var t = new StreamProtocolTest();
     t..expectListen()
      ..expectData(42, () {
          t.pause(new Future.delayed(ms5, () => null));
        })
      ..expectPause()
-     ..expectDone()
-     ..expectCancel(t.terminate);
+     ..expectCancel()
+     ..expectDone(t.terminate);
      // We are calling "close" while the controller is actually paused,
      // and it will stay paused until the pending events are sent.
     t..listen()..add(42)..close();
   });
 
   test("$p-sub-data/pause-resume/done", () {
     var t = new StreamProtocolTest();
     t..expectListen()
      ..expectData(42, () {
          t.pause(new Future.delayed(ms5, () => null));
        })
      ..expectPause()
      ..expectResume(t.close)
-     ..expectDone()
-     ..expectCancel(t.terminate);
+     ..expectCancel()
+     ..expectDone(t.terminate);
     t..listen()..add(42);
   });
 
   test("$p-sub-data/data+pause-data-resume-done", () {
     var t = new StreamProtocolTest();
     t..expectListen()
      ..expectData(42, () {
          t.add(43);
          t.pause(new Future.delayed(ms5, () => null));
          // Should now be paused until the future finishes.
          // After that, the controller stays paused until the pending queue
          // is empty.
        })
      ..expectPause()
      ..expectData(43)
      ..expectResume(t.close)
-     ..expectDone()
-     ..expectCancel(t.terminate);
+     ..expectCancel()
+     ..expectDone(t.terminate);
     t..listen()..add(42);
   });
 
   test("$p-pause-during-callback", () {
     var t = new StreamProtocolTest();
     t..expectListen()
      ..expectData(42, () {
        t.pause();
      })
      ..expectPause(() {
        t.resume();
      })
      ..expectResume(() {
        t.pause();
        t.resume();
        t.close();
      })
-     ..expectDone()
-     ..expectCancel(t.terminate);
+     ..expectCancel()
+     ..expectDone(t.terminate);
     t..listen()
      ..add(42);
   });
 }