async merge(output, wantsBabel = false) {
        /** @type {string} */
        // @ts-ignore
        const contents = await concat(this.files, output.makeDirectories().path());

        if (this.shouldCompileWithBabel(wantsBabel, output)) {
            const code = this.babelify(contents);

            if (code) {
                output.write(code);
            }
        }

        return new File(output.makeDirectories().path());
    }
Merge all files in the collection into one.

@param {File} output
@param {boolean} wantsBabel