Update stream iterator doc and add test.

sdk/lib/async/stream_impl.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;
 
 /** Abstract and private interface for a place to put events. */
 abstract class _EventSink<T> {
   void _add(T data);
   void _addError(Object error, StackTrace stackTrace);
@@ skipping 934 matching lines @@
 
   Future<bool> moveNext() {
     if (_state == _STATE_DONE) {
       return new _Future<bool>.immediate(false);
     }
     if (_state == _STATE_MOVING) {
       throw new StateError("Already waiting for next.");
     }
     if (_state == _STATE_FOUND) {
       _state = _STATE_MOVING;
+      _current = null;
       _futureOrPrefetch = new _Future<bool>();
       return _futureOrPrefetch;
     } else {
       assert(_state >= _STATE_EXTRA_DATA);
       switch (_state) {
         case _STATE_EXTRA_DATA:
           _state = _STATE_FOUND;
           _current = _futureOrPrefetch;
           _futureOrPrefetch = null;
           _subscription.resume();
@@ skipping 63 matching lines @@
       _Future<bool> hasNext = _futureOrPrefetch;
       _clear();
       hasNext._complete(false);
       return;
     }
     _subscription.pause();
     _futureOrPrefetch = null;
     _state = _STATE_EXTRA_DONE;
   }
 }