ember tests passing in chrome, not in phantomjs

I have tests for an addon which pass in chrome, but fail in phantomjs.

It seems to be a problem similar to this question. However, I tried the solution there and it didn't work for me.

The code is all available in the public repo linked above. The failures are exhibited in the failing travis build on github. Any ideas on how to diagnose better and fix?

EDIT -- actual error message

---

        Died on test #1     at http://localhost:7357/assets/test-support.js:3062
            at test (http://localhost:7357/assets/test-support.js:1945)
            at test (http://localhost:7357/assets/dummy.js:2090)
            at http://localhost:7357/assets/dummy.js:2885
            at http://localhost:7357/assets/vendor.js:150
            at tryFinally (http://localhost:7357/assets/vendor.js:30)
            at http://localhost:7357/assets/vendor.js:156
            at http://localhost:7357/assets/test-loader.js:29
            at http://localhost:7357/assets/test-loader.js:21
            at http://localhost:7357/assets/test-loader.js:40
            at http://localhost:7357/assets/test-support.js:6775: Can't find variable: Symbol