add failing test for adding in missing files

This currently fails with:
```
TypeError: Cannot read property 'getTime' of null
    at Object.File.stale (/home/swatinem/Coding/normalize/walker.js/lib/file.js:224:19)
    at GeneratorFunctionPrototype.next (native)
    at Object.file.stale (/home/swatinem/Coding/normalize/builder.js/lib/plugins/missing.js:24:31)
    at GeneratorFunctionPrototype.next (native)
    at Walker.resolve (/home/swatinem/Coding/normalize/walker.js/lib/walker.js:132:29)
```