Fix typo.

sdk/lib/async/stream_controller.dart

Index: sdk/lib/async/stream_controller.dart
diff --git a/sdk/lib/async/stream_controller.dart b/sdk/lib/async/stream_controller.dart
index 1f15703dd2853843a962623a709947828c1301e7..5e34a762990de5da4fe464d811767debfeac262c 100644
--- a/sdk/lib/async/stream_controller.dart
+++ b/sdk/lib/async/stream_controller.dart
@@ -284,7 +284,7 @@ abstract class StreamController<T> implements StreamSink<T> {
  * accidentally break other code.
  *
  * Adding events to a synchronous controller should only happen as the
- * very last part of a the handling of the original event.
+ * very last part of the handling of the original event.
  * At that point, adding an event to the stream is equivalent to
  * returning to the event loop and adding the event in the next microtask.
  *