Reapply "Active stream subscriptions".

sdk/lib/mdv_observe_impl/mdv_observe_impl.dart

Index: sdk/lib/mdv_observe_impl/mdv_observe_impl.dart
diff --git a/sdk/lib/mdv_observe_impl/mdv_observe_impl.dart b/sdk/lib/mdv_observe_impl/mdv_observe_impl.dart
index e56554bce82d3cbac789836e9b70d9b126b372f3..ed42d62ef2453b0aa663e286f361c9ea0e1546f1 100644
--- a/sdk/lib/mdv_observe_impl/mdv_observe_impl.dart
+++ b/sdk/lib/mdv_observe_impl/mdv_observe_impl.dart
@@ -63,16 +63,15 @@ typedef ObservableBase = Object with ObservableMixin;
  * call [notifyPropertyChange]. See that method for an example.
  */
 abstract class ObservableMixin implements Observable {
-  StreamController<List<ChangeRecord>> _observers;
-  Stream<List<ChangeRecord>> _stream;
+  StreamController _multiplexController;
   List<ChangeRecord> _changes;
 
   Stream<List<ChangeRecord>> get changes {
-    if (_observers == null) {
-      _observers = new StreamController<List<ChangeRecord>>();
-      _stream = _observers.stream.asBroadcastStream();
+    if (_multiplexController == null) {
+      _multiplexController =
+          new StreamController<List<ChangeRecord>>.multiplex();
     }
-    return _stream;
+    return _multiplexController.stream;
   }
 
   void _deliverChanges() {
@@ -80,7 +79,7 @@ abstract class ObservableMixin implements Observable {
     _changes = null;
     if (hasObservers && changes != null) {
       // TODO(jmesserly): make "changes" immutable
-      _observers.add(changes);
+      _multiplexController.add(changes);
     }
   }
 
@@ -88,7 +87,8 @@ abstract class ObservableMixin implements Observable {
    * True if this object has any observers, and should call
    * [notifyPropertyChange] for changes.
    */
-  bool get hasObservers => _observers != null && _observers.hasListener;
+  bool get hasObservers => _multiplexController != null &&
+                           _multiplexController.hasListener;
 
   /**
    * Notify that the field [name] of this object has been changed.