Fix definition of Array.prototype[Symbol.unscopables]

[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]

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[sandersn]

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

[ExE-Boss]

@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.

[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 @rbuckton can review it.

[MartinJohns]

Related: #46616

[graphemecluster]

Why it is still not merged? If not I am going to put this change in #49855.
@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]

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

[graphemecluster]

Conflicts again :(