Add missing null-tests to async error functions.

sdk/lib/async/stream.dart

Index: sdk/lib/async/stream.dart
diff --git a/sdk/lib/async/stream.dart b/sdk/lib/async/stream.dart
index 906476b88582cc398022675345765c7dd54ef383..d0b2f24272b8d61c89a606785d4bca011784da33 100644
--- a/sdk/lib/async/stream.dart
+++ b/sdk/lib/async/stream.dart
@@ -1380,7 +1380,7 @@ abstract class StreamSubscription<T> {
 abstract class EventSink<T> implements Sink<T> {
   /** Send a data event to a stream. */
   void add(T event);
-  /** Send an async error to a stream. */
+  /** Send an async error to a stream. The [errorEvent] must not be `null`. */
   void addError(errorEvent, [StackTrace stackTrace]);
   /** Send a done event to a stream.*/
   void close();