@sindresorhus you mean, buildnumber, usemin couldn't be targets. so then it's need to be changed as follows:
```
chromeManifest: {
  dist: {
     buildnumber: true,
     src: 'app/scripts/background.js',
     dest: 'dist/scripts/background.js'
  }
}
```
please check out this. and i've read grunt doc about multi-tasks [1]. is there more docs of grunt?

[1]: https://github.com/gruntjs/grunt/wiki/Creating-tasks#multi-tasks