Add a LazyStream class.

lib/src/subscription_stream.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.subscription_stream;
 
 import 'dart:async';
 
 import "delegate/stream_subscription.dart";
 
 /// A [Stream] adapter for a [StreamSubscription].
 ///
-/// This class allows as `StreamSubscription` to be treated as a `Stream`.
+/// This class allows a `StreamSubscription` to be treated as a `Stream`.
 ///
 /// The subscription is paused until the stream is listened to,
 /// then it is resumed and the events are passed on to the
 /// stream's new subscription.
 ///
 /// This class assumes that is has control over the original subscription.
 /// If other code is accessing the subscription, results may be unpredictable.
 class SubscriptionStream<T> extends Stream<T> {
   /// The subscription providing the events for this stream.
   StreamSubscription _source;
@@ skipping 68 matching lines @@
       } else {
         if (handleError is ZoneBinaryCallback) {
           handleError(error, stackTrace);
         } else {
           handleError(error);
         }
       }
     });
   }
 }