static preprocessor(type, src, output) {
        assert(
            typeof src === 'string',
            `mix.${type}() is missing required parameter 1: src`
        );

        assert(
            typeof output === 'string',
            `mix.${type}() is missing required parameter 2: output`
        );
    }
Assert that the calls to mix.sass() and mix.less() are valid.

@param {string} type
@param {string} src
@param {string} output