Disallow const native factories

tests/language/const_native_factory_test.dart

Index: tests/language/const_native_factory_test.dart
===================================================================
--- tests/language/const_native_factory_test.dart	(revision 0)
+++ tests/language/const_native_factory_test.dart	(revision 0)
@@ -0,0 +1,14 @@
+// Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+class Cake {
+  final name;
+  const Cake(this.name);
+  const factory BakeMeACake() native "Cake_BakeMeACake";  /// 01: compile-time error
+}
+
+main() {
+  var c = const Cake("Sacher");
+}
+