Add Iterable.fold (and Stream.fold) which replace `reduce`.

samples/swarm/swarm_ui_lib/observable/observable.dart

Index: samples/swarm/swarm_ui_lib/observable/observable.dart
diff --git a/samples/swarm/swarm_ui_lib/observable/observable.dart b/samples/swarm/swarm_ui_lib/observable/observable.dart
index 6b48637d4e9b65cd755737ff3411d274124a5887..cc48d640c8432527b638783690a12d0d4f450e53 100644
--- a/samples/swarm/swarm_ui_lib/observable/observable.dart
+++ b/samples/swarm/swarm_ui_lib/observable/observable.dart
@@ -285,6 +285,10 @@ class ObservableList<T>
     throw new UnimplementedError();
   }
 
+  dynamic fold(var initialValue,
+               dynamic combine(var previousValue, T element)) {
+    throw new UnimplementedError();
+  }
 
   // Iterable<T>:
   Iterator<T> get iterator => _internal.iterator;