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

sdk/lib/_collection_dev/list.dart

Index: sdk/lib/_collection_dev/list.dart
diff --git a/sdk/lib/_collection_dev/list.dart b/sdk/lib/_collection_dev/list.dart
index d8fe4f89a7cce1dbe39a086581baa8641e14ea52..1e40785f3a6540ab555979cb6cf7ba87fe7236d6 100644
--- a/sdk/lib/_collection_dev/list.dart
+++ b/sdk/lib/_collection_dev/list.dart
@@ -207,6 +207,10 @@ abstract class ListMixin<E> implements List<E> {
   Iterable map(f(E element)) => new MappedListIterable(this, f);
 
   reduce(var initialValue, combine(var previousValue, E element)) {
+    return fold(initialValue, combine);
+  }
+
+  fold(var initialValue, combine(var previousValue, E element)) {
     var value = initialValue;
     int length = this.length;
     for (int i = 0; i < length; i++) {