Using the plugin invalidates source maps

[spencerwilson-optimizely]

See the repo here for a minimal reproduction: https://github.com/spencerwilson-optimizely/rollup-plugin-stub-bug

It seems that the source map being emitted by the plugin isn't correct. The expected result is the same in both of the cases in the repo.

The consumer of the source map in this example is source-map-support@0.5.13.