Java: Implements union generation.

mojo/public/interfaces/bindings/tests/test_unions.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 mojo.test;
 
 enum AnEnum {
   FIRST, SECOND
 };
 
@@ skipping 37 matching lines @@
 };
 
 struct DummyStruct {
   int8 f_int8;
 };
 
 struct SmallStruct {
   DummyStruct? dummy_struct;
   PodUnion? pod_union;
   array<PodUnion>? pod_union_array;
+  array<PodUnion?>? nullable_pod_union_array;
   array<DummyStruct>? s_array;
   map<string, PodUnion>? pod_union_map;
+  map<string, PodUnion?>? nullable_pod_union_map;
 };
 
 struct SmallStructNonNullableUnion {
   PodUnion pod_union;
 };
 
 struct SmallObjStruct {
   ObjectUnion obj_union;
 };
 
 interface SmallCache {
   SetIntValue(int64 int_value);
   GetIntValue() => (int64 int_value);
 };
 
 interface UnionInterface {
   Echo(PodUnion in) => (PodUnion out);
 };