Make await for cancel the stream when breaking out of the loop

runtime/lib/core_patch.dart

Index: runtime/lib/core_patch.dart
===================================================================
--- runtime/lib/core_patch.dart	(revision 44677)
+++ runtime/lib/core_patch.dart	(working copy)
@@ -36,6 +36,7 @@
   bool isAdding = false;
   bool onListenReceived = false;
   bool isScheduled = false;
+  Completer cancellationCompleter = null;
 
   Stream get stream => controller.stream;
 
@@ -93,6 +94,12 @@
   }
 
   void addError(error, stackTrace) {
+    if ((cancellationCompleter != null) && !cancellationCompleter.isCompleted) {
+      // If the stream has been cancelled, complete the cancellation future
+      // with the error.
+      cancellationCompleter.completeError(error, stackTrace);
+      return;
+    }
     // If stream is cancelled, tell caller to exit the async generator.
     if (!controller.hasListener) return;
     controller.addError(error, stackTrace);
@@ -103,6 +110,11 @@
   }
 
   close() {
+    if ((cancellationCompleter != null) && !cancellationCompleter.isCompleted) {
+      // If the stream has been cancelled, complete the cancellation future
+      // with the error.
+      cancellationCompleter.complete();
+    }
     controller.close();
   }
 
@@ -123,7 +135,14 @@
   }
 
   onCancel() {
-    scheduleGenerator();
+    if (controller.isClosed) {
+      return null;
+    }
+    if (cancellationCompleter == null) {
+      cancellationCompleter = new Completer();
+      scheduleGenerator();
+    }
+    return cancellationCompleter.future;
   }
 }