Fix definition of Array.prototype[Symbol.unscopables]

[ExE Boss (ExE-Boss)]

#36540 was incorrectly applied to lib/es2015.symbol.wellknown.d.ts and thus got overwritten once the next TypeScript LKG lib was rebuilt.

#42566 (comment):

The second change is simply copying the Symbol.unscopables definition to ReadonlyArray, and making it so that it doesn’t need to be updated in the future, since when new methods get added to %Array.prototype%, they almost always also have to be added to %Array.prototype[Symbol.unscopables]% for compatibility with legacy code using the sloppy mode‑only with statement.

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Actually fixes #34610
Fixes #46616

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review?(@sandersn, @rbuckton)

[TypeScript Bot (typescript-bot)]

This PR doesn't have any linked issues. Please open an issue that references this PR. From there we can discuss and prioritise.

[Nathan Shively-Sanders (sandersn)]

Wait a minute. This changes more than #36540 does. What do the additional changes do?

[ExE Boss (ExE-Boss)]

Nathan Shively-Sanders (@sandersn) The second change is simply copying the Symbol.unscopables definition to ReadonlyArray, and making it so that it doesn’t need to be updated in the future, since when new methods get added to %Array.prototype%, they almost always also have to be added to %Array.prototype[Symbol.unscopables]% for compatibility with legacy code using the sloppy mode‑only with statement.

[Nathan Shively-Sanders (sandersn)]

I'd be happy to take the first two changes immediately: removing the extraneous (), and copying unscopeables' definition to ReadonlyArray. But I don't know enough about array method changes to evaluate the third. I'd recommend splitting the third change into a separate PR so Ron Buckton (@rbuckton) can review it.

[Martin Johns (MartinJohns)]

Related: #46616

[graphemecluster (graphemecluster)]

Why it is still not merged? If not I am going to put this change in #49855.
Nathan Shively-Sanders (@sandersn) [Symbol.unscopables] is mutable – try adding and deleting properties in the devtools console.
And I don't think such a small change is worth 2 PRs.

[ExE Boss (ExE-Boss)]

graphemecluster (@graphemecluster) Probably because there are merge conflicts (again).

[graphemecluster (graphemecluster)]

Conflicts again :(