Trouble with sourcemaps from istanbul-loader

**Bug**

Using `istanbul-loader` fixes the coverage report from istanbul in @dojo/widgets, but it would appear that something it's doing is messing up the source maps. @matt-gadd confirmed that the source maps are being generated, but if you open the file in Sources tab in Chrome, you'll find that the source map is empty.

** Steps to reproduce**

- From @dojo/widgets, `dojo build widget -m unit`
- Then `dojo test -c local`
- The dojo test command will print out a URL when it's finished, copy this URL!
- Start a web server from the root widgets directory
- Navigate to the URL in your clipboard
- Open up the developer tools, Sources, load a widget!