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

sdk/lib/_internal/compiler/implementation/lib/js_array.dart

Index: sdk/lib/_internal/compiler/implementation/lib/js_array.dart
diff --git a/sdk/lib/_internal/compiler/implementation/lib/js_array.dart b/sdk/lib/_internal/compiler/implementation/lib/js_array.dart
index 4dbb285eafbbe5b158d68816c2aaead49ac7fe56..c57c15b8b0a882dec53f5923b7145d627d61e062 100644
--- a/sdk/lib/_internal/compiler/implementation/lib/js_array.dart
+++ b/sdk/lib/_internal/compiler/implementation/lib/js_array.dart
@@ -125,6 +125,10 @@ class JSArray<E> extends Interceptor implements List<E>, JSIndexable {
     return IterableMixinWorkaround.reduce(this, initialValue, combine);
   }
 
+  fold(initialValue, combine(previousValue, E element)) {
+    return IterableMixinWorkaround.fold(this, initialValue, combine);
+  }
+
   E firstWhere(bool test(E value), {E orElse()}) {
     return IterableMixinWorkaround.firstWhere(this, test, orElse);
   }