[Dart] deprecated dart 2 code generated

Description

The generated dart code contains deprecations in dart 2. It should be possible to generate code without deprecation warnings. Generating valid dart 1 code should be possible as well.

Swagger-codegen version

2.4.0

Swagger declaration file content or url

http://petstore.swagger.io/v2/swagger.json

Command line used for generation

./bin/dart-petstore.sh

Steps to reproduce

• cd samples/client/petstore/dart/flutter_petstore
• flutter analyze

info • The name of the analysis options file .analysis_options is deprecated; consider renaming it to
analysis_options.yaml • swagger/.analysis_options:1:1
info • 'JSON' is deprecated and shouldn't be used • swagger/lib/api_client.dart:85:23
info • 'JSON' is deprecated and shouldn't be used • swagger/lib/api_client.dart:94:20
info • 'BASE64' is deprecated and shouldn't be used • swagger/lib/auth/http_basic_auth.dart:11:48
info • 'UTF8' is deprecated and shouldn't be used • swagger/lib/auth/http_basic_auth.dart:11:62

Flutter Version

Flutter 0.5.1 • channel beta • https://github.com/flutter/flutter.git
Framework • revision c7ea3ca377 (4 weeks ago) • 2018-05-29 21:07:33 +0200
Engine • revision 1ed25ca7b7
Tools • Dart 2.0.0-dev.58.0.flutter-f981f09760

Suggest a fix/enhancement

I suggest to add a new option for the dart version and generate code accordingly.

Start in samples/client/petstore/dart/flutter_petstore, regenerate with ./bin/dart-petstore.sh, and run flutter analyze to reproduce the listed deprecations. Review the generated files named in the report, including swagger/lib/api_client.dart, swagger/lib/auth/http_basic_auth.dart, and .analysis_options; done means generation avoids Dart 2 deprecation warnings and supports the requested Dart-version option.