[WIP] Extension methods support

[trevorwang]

#558

[trevorwang]

This broke all tests which contain json encode

The extension methods are not treated as _toEncodable functions

https://github.com/dart-lang/sdk/blob/b43714854e4d68907ab2dd033353e234a690cc6e/sdk/lib/convert/json.dart#L521

extension PersonExt on Person {
  static Person fromJson(Map<String, dynamic> json) => _$PersonFromJson(json);
  Map<String, dynamic> toJson() => _$PersonToJson(this);
}

NoSuchMethodError (NoSuchMethodError: Class 'Person' has no instance method 'toJson'.
Receiver: Instance of 'Person'
Tried calling: toJson())

[trevorwang]

Add test code here

import 'package:test/test.dart';

void main() {
  test('toJson() in extension is not visible', () async {
    final demo = Demo();
    expect(_defaultToEncodable(demo), demo.toJson());
  });
}

dynamic _defaultToEncodable(dynamic object) => object.toJson();

class Demo {}

extension DemoExt on Demo {
  Map<String, dynamic> toJson() => <String, dynamic>{'foo': 'bar'};
}

And here's the error message

Exception has occurred.
NoSuchMethodError (NoSuchMethodError: Class 'Demo' has no instance method 'toJson'.
Receiver: Instance of 'Demo'
Tried calling: toJson())

[trevorwang]

Close first due to #558 (comment)