Make new StreamController().isPaused be true until the first listener arrives.

sdk/lib/async/stream_controller.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;
 
 // -------------------------------------------------------------------
 // Controller for creating and adding events to a stream.
 // -------------------------------------------------------------------
 
@@ skipping 91 matching lines @@
   EventSink<T> get sink;
 
   /**
    * Whether the stream is closed for adding more events.
    *
    * If true, the "done" event might not have fired yet, but it has been
    * scheduled, and it is too late to add more events.
    */
   bool get isClosed;
 
-  /** Whether the subscription is active and paused. */
+  /**
+   * Whether the subscription would need to buffer events.
+   *
+   * This is the case if the controller's stream has a listener and it is
+   * paused, or if it has not received a listener yet. In that case, the
+   * controller is considered paused as well.
+   *
+   * A broadcast stream controller is never considered paused. It always
+   * forwards its events to all uncanceled listeners, if any, and let them
+   * handle their own pausing.
+   */
   bool get isPaused;
 
   /** Whether there is a subscriber on the [Stream]. */
   bool get hasListener;
 
   /**
    * Send or enqueue an error event.
    *
    * Also allows an objection stack trace object, on top of what [EventSink]
    * allows.
@@ skipping 51 matching lines @@
 
   /**
    * Whether a listener has existed and been cancelled.
    *
    * After this, adding more events will be ignored.
    */
   bool get _isCancelled => (_state & _STATE_CANCELLED) != 0;
 
   bool get isClosed => (_state & _STATE_CLOSED) != 0;
 
-  bool get isPaused => _subscription != null && _subscription._isInputPaused;
+  bool get isPaused => hasListener ? _subscription._isInputPaused
+                                   : !_isCancelled;
 
   bool get hasListener => _subscription != null;
 
   /**
    * Send or queue a data event.
    */
   void add(T value) {
     if (isClosed) throw new StateError("Adding event after close");
     if (_subscription != null) {
       _subscription._add(value);
@@ skipping 352 matching lines @@
         link = subscription._next;
       }
     }
     _state &= ~_STATE_FIRING;
 
     if (_isEmpty) {
       _runGuarded(_onCancel);
     }
   }
 }