Implementation of async-await transformation on js ast.

sdk/lib/async/stream_impl.dart

Index: sdk/lib/async/stream_impl.dart
diff --git a/sdk/lib/async/stream_impl.dart b/sdk/lib/async/stream_impl.dart
index 394e1c6a7c72b51b25fc7df35554941a94e2e2e5..eef9660404d6261e713bc8424eef0a4d01de0958 100644
--- a/sdk/lib/async/stream_impl.dart
+++ b/sdk/lib/async/stream_impl.dart
@@ -1036,7 +1036,10 @@ class _StreamIteratorImpl<T> implements StreamIterator<T> {
     } else {
       _clear();
     }
-    return subscription.cancel();
+    // If the stream is done, the subscription will have been cleared.
+    return subscription == null
+        ? new _Future<Null>.immediate(null)
+        : subscription.cancel();
   }
 
   void _onData(T data) {