This test has not been working correctly since at least a555be2.

Since it tests internals, any replacement test might become invalid
in a similar way.

Enable `NodeEventTarget` as a base class for `MessagePort`,
by enabling special processing of events for Node.js listeners,
and removing implicit constructors/private properties so that
classes can be made subclasses of `NodeEventTarget` after they
are created.

The messaging code uses `Object.defineProperty()`, which accesses
`value` on `Object.prototype` by default, so some calls to the
getter here would actually be expected. Instead, make the list
of accessed properties more specific to the tested source map
code to avoid flakiness.

Refs: https://twitter.com/addaleax/status/1276289101671608320

Use `NodeEventTarget` to provide a mixed `EventEmitter`/`EventTarget`
API interface.