Blueprints: filesTree seems requires a resource property

This step should work without a resource property for filesTree

{
    "$schema": "https://playground.wordpress.net/blueprint-schema.json",
    "steps": [
    {
        "step": "writeFiles",
            "writeToPath": "/wordpress/wp-content/plugins/woo-shipping-method",
            "filesTree": {
            "name": "woo-shipping-method",
            "files": {
                "plugin.php": "<?php\n/*\nPlugin Name: My Plugin\n*/",
                "readme.txt": "=== My Plugin ===\nDescription: A simple plugin",
                "includes": {
                    "class-main.php": "<?php\nclass Main_Class {}",
                    "functions.php": "<?php\nfunction my_plugin_init() {}"
                }
           }
      }
]
}

Message in Browser console:

Error: Invalid blueprint: must have required property 'resource' at /steps/0/filesTree

Start by locating the blueprint schema and validation for the writeFiles step's filesTree property, then reproduce the supplied blueprint and browser-console error. Done means the example is accepted without a resource property and existing filesTree validation still passes.