## Current Behavior
Rollup build fails on Windows with errors like this:
```
[plugin rpt2] error TS6059: File '/foo/bar/baz.ts' is not under 'rootDir' 'C:/foo/bar'. 'rootDir' is expected to contain all source files.
```
This is because since
81fe7bb278
the `tsconfig` path passed to `rollup-plugin-typescript2` is built using
`joinPathFragments`, which removes the drive letter on Windows.
## Expected Behavior
Rollup build should not fail.
659149d87c