Check for the presence of all required executables as soon as possible (before artifacts are downloaded) in the tool and give a good error message. flutter doctor makes sense but the validators may be run too late to catch git or zip exceptions.

https://flutter.dev/docs/get-started/install/windows#system-requirements

• Windows PowerShell 5.0 or newer (this is pre-installed with Windows 10)
• Git for Windows 2.x, with the Use Git from the Windows Command Prompt option.

https://flutter.dev/docs/get-started/install/macos#system-requirements

• bash
• curl
• git 2.x
• mkdir
• rm
• unzip
• which
• zip

https://flutter.dev/docs/get-started/install/linux#system-requirements
(above list + xz-utils)