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

runtime/lib/array.dart

Index: runtime/lib/array.dart
diff --git a/runtime/lib/array.dart b/runtime/lib/array.dart
index 696bfcc5adca81966fdcd329fb5204903ac05ec5..e92744c745ae0d5c62f17922c39976b6bbfafb27 100644
--- a/runtime/lib/array.dart
+++ b/runtime/lib/array.dart
@@ -118,6 +118,10 @@ class _ObjectArray<E> implements List<E> {
     return IterableMixinWorkaround.reduce(this, initialValue, combine);
   }
 
+  fold(initialValue, combine(previousValue, E element)) {
+    return IterableMixinWorkaround.fold(this, initialValue, combine);
+  }
+
   Iterable<E> where(bool f(E element)) {
     return IterableMixinWorkaround.where(this, f);
   }
@@ -361,6 +365,10 @@ class _ImmutableArray<E> implements List<E> {
     return IterableMixinWorkaround.reduce(this, initialValue, combine);
   }
 
+  fold(initialValue, combine(previousValue, E element)) {
+    return IterableMixinWorkaround.fold(this, initialValue, combine);
+  }
+
   Iterable<E> where(bool f(E element)) {
     return IterableMixinWorkaround.where(this, f);
   }