The following used to fail in Deno despite working in the browser:
```javascript
new Request('http://localhost/', {method: 'POST', body: new URLSearchParams({hello: 'world'})}).text().then(console.log)
```
Refactors Event and EventTarget so that they better encapsulate their
non-public data as well as are more forward compatible with things like
DOM Nodes.
Moves `dom_types.ts` -> `dom_types.d.ts` which was always the intention,
it was a legacy of when we used to build the types from the code and the
limitations of the compiler. There was a lot of cruft in `dom_types`
which shouldn't have been there, and mis-alignment to the DOM standards.
This generally has been eliminated, though we still have some minor
differences from the DOM (like the removal of some deprecated
methods/properties).
Adds `DOMException`. Strictly it shouldn't inherit from `Error`, but
most browsers provide a stack trace when one is thrown, so the behaviour
in Deno actually better matches the browser.
`Event` still doesn't log to console like it does in the browser. I
wanted to get this raised and that could be an enhancement later on (it
currently doesn't either).
