Add missing null-tests to async error functions.

sdk/lib/async/stream_controller.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 part of dart.async;
 
 // -------------------------------------------------------------------
 // Controller for creating and adding events to a stream.
 // -------------------------------------------------------------------
 
@@ skipping 150 matching lines @@
    * handle their own pausing.
    */
   bool get isPaused;
 
   /** Whether there is a subscriber on the [Stream]. */
   bool get hasListener;
 
   /**
    * 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.
    */
   void addError(Object error, [StackTrace stackTrace]);
 
   /**
    * Receives events from [source] and puts them into this controller's stream.
    *
    * Returns a future which completes when the source stream is done.
    *
@@ skipping 226 matching lines @@
    */
   void add(T value) {
     if (!_mayAddEvent) throw _badEventState();
     _add(value);
   }
 
   /**
    * 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);
   }
 
   /**
    * Closes this controller and sends a done event on the stream.
    *
    * The first time a controller is closed, a "done" event is added to its
    * stream.
@@ skipping 360 matching lines @@
   _StreamControllerAddStreamState(_StreamController controller,
                                   this.varData,
                                   Stream source,
                                   bool cancelOnError)
       : super(controller, source, cancelOnError) {
     if (controller.isPaused) {
       addSubscription.pause();
     }
   }
 }