Context: whatwg/html#3195
Context: #202

FormData should have ability to add an entry for a submitter button when it appends entries for a <form>.

We have multiple options of how to specify submitter. See five comments since whatwg/html#3195 (comment)

A) constructor(optional (HTMLFormElement or record<USVString, FormDataEntryValue>) formOrMap, optional HTMLElement? submitter = null)
Idiomatic.

B) constructor(optional (HTMLElement or record<...>) formOrSubmitterOrMap)
HTMLElement represents a form or a submitter.

C) constructor(optional (HTMLFormElement or FormDataInit) formOrDict)

dictionary FormDataInit {
 HTMLFormElement form;
 HTMLElement? submitter;
 record<...> map;
}

The content of the dictionary is idiomatic.
Need to wrap a record with a dictionary.
Extensible. It's easy to add new members to the dictionary in the future.

D) constructor(optional (HTMLFormElement or FormDataInit or URLSearchParams) init)

dictionary FormDataInit {
  required HTMLFormElement form;
  HTMLElement? submitter = null;
}

FormData doesn't support record<> directly. Developers have to write new FormData(new URLSearchParams(map)).

E) constructor(optional HTMLFormElement form, optional HTMLElement? submitter = null)
FormData append(record<USVString, FormDataEntryValue> map);
Developers have to write new FormData().append(map).

F) (no changes on the constructor)

FormData append(HTMLFormElement form, optional HTMLElement? submitter = null);
FormData append(record<USVString, FormDataEntryValue> map);

Developers have to write let fd = new FormData().append(form, submitter); if they want to collect entries including an entry for the submitter.