Add missing null-tests to async error functions.

sdk/lib/async/stream.dart

 // Copyright (c) 2013, 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;
 
 // -------------------------------------------------------------------
 // Core Stream types
 // -------------------------------------------------------------------
 
@@ skipping 1364 matching lines @@
 
 
 /**
  * An interface that abstracts creation or handling of [Stream] events.
  */
 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. */
   void addError(errorEvent, [StackTrace stackTrace]);
-  /** Send a done event to a stream.*/
+  /** Send a done event to a stream. */
   void close();
 }
 
 
 /** [Stream] wrapper that only exposes the [Stream] interface. */
 class StreamView<T> extends Stream<T> {
   Stream<T> _stream;
 
   StreamView(this._stream);
 
@@ skipping 240 matching lines @@
 class _ControllerEventSinkWrapper<T> implements EventSink<T> {
   EventSink _sink;
   _ControllerEventSinkWrapper(this._sink);
 
   void add(T data) { _sink.add(data); }
   void addError(error, [StackTrace stackTrace]) {
     _sink.addError(error, stackTrace);
   }
   void close() { _sink.close(); }
 }