Cancel async generator only when it yields

runtime/lib/async_patch.dart

 // Copyright (c) 2015, 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.
 
 import "dart:_internal";
 
 // We need to pass the value as first argument and leave the second and third
 // arguments empty (used for error handling).
 // See vm/ast_transformer.cc for usage.
 Function _asyncThenWrapperHelper(continuation) {
@@ skipping 68 matching lines @@
   }
 
   void scheduleGenerator() {
     if (isScheduled || controller.isPaused || isAdding) {
       return;
     }
     isScheduled = true;
     scheduleMicrotask(runBody);
   }
 
-  // Adds element to steam, returns true if the caller should terminate
+  // Adds element to stream, returns true if the caller should terminate
   // execution of the generator.
   //
   // TODO(hausner): Per spec, the generator should be suspended before
   // exiting when the stream is closed. We could add a getter like this:
   // get isCancelled => controller.hasListener;
   // The generator would translate a 'yield e' statement to
   // controller.add(e);
   // suspend;
   // if (controller.isCancelled) return;
   bool add(event) {
@@ skipping 70 matching lines @@
       scheduleGenerator();
     }
   }
 
   onCancel() {
     if (controller.isClosed) {
       return null;
     }
     if (cancellationCompleter == null) {
       cancellationCompleter = new Completer();
-      scheduleGenerator();
+      // Only resume the generator if it is suspended at a yield.
+      // Cancellation does not affect an async generator that is
+      // suspended at an await.
+      if (isSuspendedAtYield) {
+        scheduleGenerator();
+      }
     }
     return cancellationCompleter.future;
   }
 }
 
 patch void _rethrow(Object error, StackTrace stackTrace) native "Async_rethrow";