Update Stream.listen doc and use ?? in some places.

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 307 matching lines @@
    * is called. If [onData] is null, nothing happens.
    *
    * On errors from this stream, the [onError] handler is given a
    * object describing the error.
    *
    * The [onError] callback must be of type `void onError(error)` or
    * `void onError(error, StackTrace stackTrace)`. If [onError] accepts
    * two arguments it is called with the stack trace (which could be `null` if
    * the stream itself received an error without stack trace).
    * Otherwise it is called with just the error object.
+   * If [onError] is omitted, any errors on the stream are considered unhandled,
+   * and will be passed to the current [Zone]'s error handler.
+   * By default unhandled async errors are treated
+   * as if they were uncaught top-level errors.
    *
    * If this stream closes, the [onDone] handler is called.
    *
    * If [cancelOnError] is true, the subscription is ended when
    * the first error is reported. The default is false.
    */
   StreamSubscription<T> listen(void onData(T event),
                                { Function onError,
                                  void onDone(),
                                  bool cancelOnError});
@@ skipping 1454 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(); }
 }