Dart: Fixes enum array encoding

mojo/public/interfaces/bindings/tests/regression_tests.mojom

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // Module containing entities for regression tests of the generator. Entities
 // must never be modified, instead new entity must be added to add new tests.
 [DartPackage="_mojo_for_test_only",
  JavaPackage="org.chromium.mojo.bindings.test.mojom.regression_tests"]
 module regression_tests;
 
@@ skipping 71 matching lines @@
 
 enum NormalEnum {
   FIRST,
   SECOND,
 };
 
 struct ContainsArrayOfEnum {
   array<NormalEnum> array_of_enums;
 };
 
+struct ContainsNullableArrayOfEnum {
+  array<NormalEnum>? array_of_enums;
+};
+
 interface Regression551 {
   Get(array<string> key_prefixes) => (int32 result);
 };
 
 interface ServiceName {
   serviceName() => (string serviceName);
 };
 
 enum CamelCaseTestEnum {
   BOOL_THING,
   DOUBLE_THING,
   FLOAT_THING,
   INT8_THING,
   INT16THING,
   INT32_TH1NG,
   INT64_Th1ng,
   UINT8T_h1ng,
   UINT16t_h1NG,
   UINT32_TH1ng,
   UINT64_TH1NG,
 };