title: Flutter JSON Serialization - Not generating *.g.dart files
body: <p>I am new to flutter and the objective is to serialise complex JSON objects which contain other smaller objects.</p>
  
  <p>Using the <code>json_serializable: ^2.0.0</code> and the <code>pubspec.yaml</code> file looks something like this.</p>
  
  <pre><code>dependencies:
    intl: ^0.15.7
    json_annotation: ^2.0.0
    built_value: ^6.7.1
    flutter:
      sdk: flutter
  
  dev_dependencies:
    build_runner: ^1.0.0
    json_serializable: ^2.0.0
    built_value_generator: ^6.7.1
    flutter_test:
      sdk: flutter
  </code></pre>
  
  <p>The <code>user.dart</code> look like this</p>
  
  <pre><code>import 'package:json_annotation/json_annotation.dart';
  
  part 'user.g.dart';
  
  @JsonSerializable(nullable: false)
  class User {
    final String firstName;
    final String lastName;
    final DateTime dateOfBirth;
    User({this.firstName, this.lastName, this.dateOfBirth});
    factory User.fromJson(Map&lt;String, dynamic&gt; json) =&gt; _$UserFromJson(json);
    Map&lt;String, dynamic&gt; toJson() =&gt; _$UserToJson(this);
  }
  </code></pre>
  
  <p>I have tried <code>flutter pub run build_runner build</code> yet the file user.g.dart doesn't get created and I am facing issue with that.</p>
  
  <p>I also have added the <code>build.yaml</code> file with following code</p>
  
  <pre><code>targets:
    $default:
      builders:
        built_value_generator|built_value:
          generate_for:
            - model/*.dart
        json_serializable|json_serializable:
          generate_for:
            - model/*.dart
  </code></pre>
  
  <p>Can anyone let me know what I am missing here. Thanks</p>