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

sdk/lib/_collection_dev/iterable.dart

Index: sdk/lib/_collection_dev/iterable.dart
diff --git a/sdk/lib/_collection_dev/iterable.dart b/sdk/lib/_collection_dev/iterable.dart
index 25515a9f9a63f65a5d948ff78162a9bac32458bc..e8a6a956ac1fbb62d61f8260cf789b694dc8e042 100644
--- a/sdk/lib/_collection_dev/iterable.dart
+++ b/sdk/lib/_collection_dev/iterable.dart
@@ -200,6 +200,10 @@ abstract class ListIterable<E> extends Iterable<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++) {
@@ -670,6 +674,10 @@ class EmptyIterable<E> extends Iterable<E> {
   Iterable map(f(E element)) => const EmptyIterable();
 
   reduce(var initialValue, combine(var previousValue, E element)) {
+    return fold(initialValue, combine);
+  }
+
+  fold(var initialValue, combine(var previousValue, E element)) {
     return initialValue;
   }