Update StreamTransformer documentation.

sdk/lib/async/stream_transformers.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;
 
 /**
  * Wraps an [_EventSink] so it exposes only the [EventSink] interface.
  */
 class _EventSinkWrapper<T> implements EventSink<T> {
@@ skipping 258 matching lines @@
  * Instead of implementing three classes: a [StreamTransformer], a [Stream]
  * (as the result of a `bind` call) and a [StreamSubscription] (which does the
  * actual work), this class only requires a function that is invoked when the
  * last bit (the subscription) of the transformer-workflow is needed.
  *
  * The given transformer function maps from Stream and cancelOnError to a
  * `StreamSubscription`. As such it can also act on `cancel` events, making it
  * fully general.
  */
 class _StreamSubscriptionTransformer<S, T> implements StreamTransformer<S, T> {
-  final _SubscriptionTransformer<S, T> _transformer;
+  final _SubscriptionTransformer<S, T> _onListen;
 
-  const _StreamSubscriptionTransformer(this._transformer);
+  const _StreamSubscriptionTransformer(this._onListen);
 
   Stream<T> bind(Stream<S> stream) =>
-      new _BoundSubscriptionStream<S, T>(stream, _transformer);
+      new _BoundSubscriptionStream<S, T>(stream, _onListen);
 }
 
 /**
  * A stream transformed by a [_StreamSubscriptionTransformer].
  *
- * When this stream is listened to it invokes the [_transformer] function with
+ * When this stream is listened to it invokes the [_onListen] function with
  * the stored [_stream]. Usually the transformer starts listening at this
  * moment.
  */
 class _BoundSubscriptionStream<S, T> extends Stream<T> {
-  final _SubscriptionTransformer<S, T> _transformer;
+  final _SubscriptionTransformer<S, T> _onListen;
   final Stream<S> _stream;
 
-  _BoundSubscriptionStream(this._stream, this._transformer);
+  _BoundSubscriptionStream(this._stream, this._onListen);
 
   StreamSubscription<T> listen(void onData(T event),
       {Function onError, void onDone(), bool cancelOnError}) {
     cancelOnError = identical(true, cancelOnError);
-    StreamSubscription<T> result = _transformer(_stream, cancelOnError);
+    StreamSubscription<T> result = _onListen(_stream, cancelOnError);
     result.onData(onData);
     result.onError(onError);
     result.onDone(onDone);
     return result;
   }
 }