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

tests/corelib/collection_test.dart

Index: tests/corelib/collection_test.dart
diff --git a/tests/corelib/collection_test.dart b/tests/corelib/collection_test.dart
index 8ef81f7a0427de71f36e339c918ee3233e22c992..86f1114855f5e4afa927882868782840a65ab63d 100644
--- a/tests/corelib/collection_test.dart
+++ b/tests/corelib/collection_test.dart
@@ -8,13 +8,12 @@ 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));
   }
 }