Adding JSON example. (#1)

jsonexample/lib/built_value/built_simple_object.dart

@@ -0,0 +1,35 @@
+// Copyright 2018 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.
+
+import 'package:built_collection/built_collection.dart';
+import 'package:built_value/built_value.dart';
+import 'package:built_value/serializer.dart';
+
+part 'built_simple_object.g.dart';
+
+abstract class BuiltSimpleObject
+    implements Built<BuiltSimpleObject, BuiltSimpleObjectBuilder> {
+  static Serializer<BuiltSimpleObject> get serializer =>
+      _$builtSimpleObjectSerializer;
+
+  @nullable
+  String get aString;
+
+  @nullable
+  int get anInt;
+
+  @nullable
+  double get aDouble;
+
+  BuiltList<String> get aListOfStrings;
+
+  BuiltList<int> get aListOfInts;
+
+  BuiltList<double> get aListOfDoubles;
+
+  BuiltSimpleObject._();
+
+  factory BuiltSimpleObject([updates(BuiltSimpleObjectBuilder b)]) =
+      _$BuiltSimpleObject;
+}