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

runtime/lib/growable_array.dart

Index: runtime/lib/growable_array.dart
diff --git a/runtime/lib/growable_array.dart b/runtime/lib/growable_array.dart
index bf52c4385baa5834baeebbf9c7134df79a5366e5..753ba1e5a925b32657517505772a91ce5bddca63 100644
--- a/runtime/lib/growable_array.dart
+++ b/runtime/lib/growable_array.dart
@@ -268,6 +268,10 @@ class _GrowableObjectArray<T> implements List<T> {
     return IterableMixinWorkaround.reduce(this, initialValue, combine);
   }
 
+  fold(initialValue, combine(previousValue, T element)) {
+    return IterableMixinWorkaround.fold(this, initialValue, combine);
+  }
+
   Iterable<T> where(bool f(T element)) {
     return IterableMixinWorkaround.where(this, f);
   }