Add a LazyStream class.

lib/src/stream_splitter.dart

 // Copyright (c) 2015, 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.
 
 library async.stream_splitter;
 
 import 'dart:async';
-import 'dart:collection';
 
 import '../result.dart';
 import 'future_group.dart';
 
 /// A class that splits a single source stream into an arbitrary number of
 /// (single-subscription) streams (called "branch") that emit the same events.
 ///
 /// Each branch will emit all the same values and errors as the source stream,
 /// regardless of which values have been emitted on other branches. This means
 /// that the splitter stores every event that has been emitted so far, which may
@@ skipping 185 matching lines @@
   }
 
   /// Marks [_controllers] as done.
   void _onDone() {
     _isDone = true;
     for (var controller in _controllers) {
       _closeGroup.add(controller.close());
     }
   }
 }