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

samples/swarm/swarm_ui_lib/observable/observable.dart

 // Copyright (c) 2011, 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.
 
 library observable;
 
 import 'dart:collection';
 
 part 'ChangeEvent.dart';
 part 'EventBatch.dart';
@@ skipping 267 matching lines @@
 
   bool contains(T element) {
     throw new UnimplementedError();
   }
 
   dynamic reduce(var initialValue,
                  dynamic combine(var previousValue, T element)) {
     throw new UnimplementedError();
   }
 
+  dynamic fold(var initialValue,
+               dynamic combine(var previousValue, T element)) {
+    throw new UnimplementedError();
+  }
 
   // Iterable<T>:
   Iterator<T> get iterator => _internal.iterator;
 
   // Collection<T>:
   Iterable<T> where(bool f(T element)) => _internal.where(f);
   Iterable map(f(T element)) => _internal.map(f);
   Iterable expand(Iterable f(T element)) => _internal.expand(f);
   List<T> skip(int count) => _internal.skip(count);
   List<T> take(int count) => _internal.take(count);
@@ skipping 37 matching lines @@
     // Only fire on an actual change.
     if (!identical(newValue, _value)) {
       final oldValue = _value;
       _value = newValue;
       recordPropertyUpdate("value", newValue, oldValue);
     }
   }
 
   T _value;
 }