Reproduction of a possible bug in karma-runner/karma with restartOnFileChange: true.

(See previous occurrence of a similar bug back in 2015)

Steps to reproduce:

1. Clone this repo (git clone https://github.com/scottohara/karma-restartonfilechange-bug)
2. Change into directory (cd karma-restartonfilechange-bug)
3. Install npm dependencies (npm install)
4. Start karma (karma start)
5. Open the test.js file, and repeatedly hit save. Note that the test successfully re-runs on each save.
6. Stop karma (CTRL-C)
7. Edit karma.conf.js and set restartOnFileChange: true
8. Start karma (karma start)
9. Open the test.js file, and repeatedly hit save. Note that the test does not re-run on each save.