resources/dispatcher.js provides send() and receive(). This is an universal message passing API. It works cross-origin, and even access different browser context groups. Messages are queued, this means one doesn't need to wait for the receiver to listen, before sending the first message.
resources/executor.html is a document. Test can send arbitrary javascript to evaluate in its execution context. This is universal and avoids introducing many specific XXX-helper.html resources. Moreover, tests are easier to read, because the whole logic of the test can be defined in a single file.