可以在bitrise.yml文件中指定使用的Flutter和Dart版本，以确保构建使用最新的版本。 示例代码：

workflows: My-Workflow: environment: FLUTTER_VERSION: "beta" DART_VERSION: "2.9.0"

steps:
- activate-ssh-key@4.0.3:
    run_if: '{{getenv "SSH_RSA_PRIVATE_KEY" | ne ""}}'
- git-clone@4.0.11: {}
- flutter-installer@0.9.0:
    inputs:
    - stable_channel: 'true'
      channel_beta: 'true'
- flutter-sdk@1.0.1:
    inputs:
    - flutter_version: "{{ env.FLUTTER_VERSION }}"
- dart-sdk@1.0.2:
    inputs:
    - dart_version: "{{ env.DART_VERSION }}" 
- script@1.1.5:
    inputs:
    - content: flutter pub get
- script@1.1.5:
    inputs:
    - content: flutter build apk --release --flavor flavorName

这里用了环境变量FLUTTER_VERSION和DART_VERSION来指定Flutter和Dart使用的版本，然后在相应的步骤中传递给了flutter-sdk和dart-sdk。这样就能确保Bitrise使用指定的版本进行构建。