replace a stray reference to runfiles to use rootDir()
replace a stray reference to runfiles to use rootDir()

I refactored this code to rely on a rootDir() helper, but overlooked
one user.  Making all code use the same helper helps with importing
this code into Google, because we need to remap all the paths.

angular/tsickle,alexeagle/tsickle,alexeagle/tsickle,alexeagle/tsickle,angular/tsickle