How do I test services in polymer?

I'm writing a polymer app and have a service that is used by polymer elements. I want to test this service but can't figure out how.

Here's what I have in the service:

<script src="../../webcomponentsjs/webcomponents-lite.js"></script>
<script src="../../web-component-tester/browser.js"></script>

<link rel="import" href="../test-service.html">

<script>
define('test-service', () => {
  class TestService {
    constructor(componentA, componentB) {
      this.componentA = componentA;
      this.componentB = componentB;
    }
  }
  return TestService;
});
</script>

How can I test this? If I try simply including the .html file I don't have access to TestService.