static combine(src, output) {
        assert(
            typeof src === 'string' || Array.isArray(src),
            `mix.combine() requires a valid src string or array.`
        );

        assert(
            output.isFile(),
            'mix.combine() requires a full output file path as the second argument. Got ' +
                output.path()
        );
    }
Assert that calls to mix.combine() are valid.

@param {string|string[]} src
@param {File}   output