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

tests/corelib/collection_test.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 collection_test;
 
 import 'dart:collection' show Queue;
 
 class CollectionTest {
   CollectionTest(Collection collection) {
-    testReduce(collection);
+    testFold(collection);
   }
 
-  void testReduce(Collection collection) {
-    Expect.equals(28, collection.reduce(0, (prev, element) => prev + element));
-    Expect.equals(
-        3024, collection.reduce(1, (prev, element) => prev * element));
+  void testFold(Collection collection) {
+    Expect.equals(28, collection.fold(0, (prev, element) => prev + element));
+    Expect.equals(3024, collection.fold(1, (prev, element) => prev * element));
   }
 }
 
 
 main() {
   final TEST_ELEMENTS = const [4, 2, 6, 7, 9];
   // Const list.
   new CollectionTest(TEST_ELEMENTS);
 
   // Fixed size list.
   var fixedList = new List(TEST_ELEMENTS.length);
   for (int i = 0; i < TEST_ELEMENTS.length; i++) {
     fixedList[i] = TEST_ELEMENTS[i];
   }
   new CollectionTest(fixedList);
 
   // Growable list.
   new CollectionTest(new List.from(TEST_ELEMENTS));
 
   // Set.
   new CollectionTest(new Set.from(TEST_ELEMENTS));
 
   // Queue.
   new CollectionTest(new Queue.from(TEST_ELEMENTS));
 }