Disallow non-attribute frozen arrays

[domenic]

Also an an example of how to specify frozen array attributes.

Closes #1399.

• At least two implementers are interested (and none opposed):
  • Should be compatible with all code
• Tests are written and can be reviewed and commented upon at:
  • Not needed
• Implementation bugs are filed:
  • Chromium: https://issues.chromium.org/issues/350519230
  • Gecko: https://bugzilla.mozilla.org/show_bug.cgi?id=1905806
  • WebKit: https://bugs.webkit.org/show_bug.cgi?id=276108
  • Deno: N/A
  • Node.js: N/A
  • webidl2.js: BREAKING CHANGE: disallow FrozenArray<T> in non-attribute position w3c/webidl2.js#791
  • widlparser: Disallow FrozenArray<T> in non-attribute position plinss/widlparser#87
• MDN issue is filed: N/A
• The top of this comment includes a clear commit message to use.

(See WHATWG Working Mode: Changes for more details.)

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[annevk]

This just works? So if it's not frozen it ends up throwing or some such?

[domenic]

The conversion steps for JS values to Web IDL FrozenArray<T> types will take any iterable, iterate over it, grab the values into an Infra list, then create a JS array from them, then freeze it, then package it up into a FrozenArray<T> pointer. So yeah, it just works.