[Dart 2] Add support for Dart 2 (#754)

* Add an option for Dart2

* add dart2 samples, update travis

* fix dart installation

* Upper constraints on the SDK version

* Update dependencies

* supportDart2 option can now be passed through --additional-properties

* Update petstore tests

* Update dart2-petstore.sh

* Running tests on Dart VM

* Fixed JSON deserialization bugs

* Fixed missing initialization of postBody

* Run bin/dart2-petstore.sh to regenerate libraries

* Update pom.xml

* Added SDK version constraints in pubspec.mustache

* Run bin/dart2-petstore.sh to regenerate libraries

* move dart2 test to the end

samples/client/petstore/dart2/openapi/lib/api.dart

@@ -0,0 +1,27 @@
+library openapi.api;
+
+import 'dart:async';
+import 'dart:convert';
+import 'package:http/http.dart';
+
+part 'api_client.dart';
+part 'api_helper.dart';
+part 'api_exception.dart';
+part 'auth/authentication.dart';
+part 'auth/api_key_auth.dart';
+part 'auth/oauth.dart';
+part 'auth/http_basic_auth.dart';
+
+part 'api/pet_api.dart';
+part 'api/store_api.dart';
+part 'api/user_api.dart';
+
+part 'model/api_response.dart';
+part 'model/category.dart';
+part 'model/order.dart';
+part 'model/pet.dart';
+part 'model/tag.dart';
+part 'model/user.dart';
+
+
+ApiClient defaultApiClient = ApiClient();