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..e51e86528601b3cae4db9a4b231a9bb4b1996ab4 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.
    *
+   * The [error] must not be `null`.
+   *
    * Also allows an objection stack trace object, on top of what [EventSink]
    * allows.
    */
@@ -414,10 +416,12 @@ abstract class _StreamController<T> implements StreamController<T>,
    * Send or enqueue an error event.
    */
   void addError(Object error, [StackTrace stackTrace]) {
+    if (error == null) throw new ArgumentError("Error must not be null");
     if (!_mayAddEvent) throw _badEventState();
     AsyncError replacement = Zone.current.errorCallback(error, stackTrace);
     if (replacement != null) {
       error = replacement.error;
+      if (error == null) error = new NullThrownError();
       stackTrace = replacement.stackTrace;
     }
     _addError(error, stackTrace);