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

runtime/lib/typeddata.dart

Index: runtime/lib/typeddata.dart
diff --git a/runtime/lib/typeddata.dart b/runtime/lib/typeddata.dart
index 4375a1c5ce3ea9a8e4e83fadfad5e9aad02b2712..8ea689ae2ee7cf36975f9bc25e9a74a2ff0fd052 100644
--- a/runtime/lib/typeddata.dart
+++ b/runtime/lib/typeddata.dart
@@ -312,6 +312,11 @@ abstract class _TypedListBase {
     return IterableMixinWorkaround.reduce(this, initialValue, combine);
   }
 
+  dynamic fold(dynamic initialValue,
+                 dynamic combine(dynamic initialValue, element)) {
+    return IterableMixinWorkaround.fold(this, initialValue, combine);
+  }
+
   Collection where(bool f(int element)) {
     return IterableMixinWorkaround.where(this, f);
   }