Add missing null-tests to async error functions.

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 fe3fbddc72973bff4d4a5d909155a5da1540c08f..aa45876e3286ad44b351d19acdaa0bc7a3f6b232 100644
--- a/sdk/lib/async/stream_controller.dart
+++ b/sdk/lib/async/stream_controller.dart
@@ -168,6 +168,8 @@ abstract class StreamController<T> implements StreamSink<T> {
   /**
    * Send or enqueue an error event.
    *
+   * If [error] is `null`, it is replaced by a [NullThrownError].
+   *
    * Also allows an objection stack trace object, on top of what [EventSink]
    * allows.
    */
@@ -414,10 +416,11 @@ abstract class _StreamController<T> implements StreamController<T>,
    * Send or enqueue an error event.
    */
   void addError(Object error, [StackTrace stackTrace]) {
+    error = _nonNullError(error);
     if (!_mayAddEvent) throw _badEventState();
     AsyncError replacement = Zone.current.errorCallback(error, stackTrace);
     if (replacement != null) {
-      error = replacement.error;
+      error = _nonNullError(replacement.error);
       stackTrace = replacement.stackTrace;
     }
     _addError(error, stackTrace);