Flutter not building - seems like codegen doesn't support Dart 2.1

Description

Trying to build code generated by codegen for Dart/Flutter project and getting the following error:

The current Dart SDK version is 2.1.2-dev.0.0.flutter-0a7dcf17eb.
Because MeBa depends on swagger from path which requires SDK version <2.0.0, version solving failed.
pub get failed (1)

Swagger-codegen version

2.4.5 (latest as of 3-Feb-2019)

Swagger declaration file content or url

My Flutter pubspec:
YAML:
name: MeBa
description: My First Flutter project
version: 1.0.0+1

environment:
sdk: ">=2.0.0-dev.68.0 <3.0.0"

dependencies:
flutter:
sdk: flutter

cupertino_icons: ^0.1.2

english_words: ^3.1.0
http: ^0.12.0+1
google_sign_in: ^4.0.1+1
firebase_auth: ^0.7.0
cloud_firestore:
date_format: ^1.0.6
font_awesome_flutter: ^8.4.0
flutter_datetime_picker: ^1.1.3
shared_preferences: ^0.5.1+2
json_annotation: ^2.2.0
build_runner: ^1.2.0
swagger:
path: ./swagger

dev_dependencies:
json_serializable: ^2.1.2
flutter_test:
sdk: flutter

flutter:
uses-material-design: true

assets:
- assets/env/dev.json

Command line used for generation

flutter packages get build

Steps to reproduce

Copy files generated by codegen (I used the run-in-docker command) to the project root directory, under "swagger". Link to it under dependencies/swagger, and run build.

Related issues/PRs

Suggest a fix/enhancement

I am trying to understand where the Dart limitation, to be <2 is defined. I checked the one dependency Swagger has, http, and it looks up to date.

Thanks in advance.

Reproduce the failure using the Flutter pubspec shown in the issue, the generated files under the swagger path dependency, and the flutter packages get build command. Start by inspecting the generated Dart package metadata and its SDK constraint; done means the generated project resolves successfully with the reported Dart 2.1 Flutter SDK.