Fix legacy platform object [[Set]] operation

[ExE-Boss]

This fixes the legacy platform object [[Set]] operation implementation to match the WebIDL spec, which doesn’t exclude array index properties from named properties when indexed setters aren't supported.

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It also fixes the bug that was caused by the receiver argument in a Proxy trap pointing to the Proxy exotic object or an object that inherits from it, rather than to the underlying Proxy target.

This can be verified with:

(new Proxy({}, { get(target,_,receiver) { console.log(target === receiver); } })).foo;

Which logs false.

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review?(@domenic)

[domenic]

What does this impact? Can we add a web platform test for it?

[ExE-Boss]

It causes the webstorage/set.window.js test to fail with 4 failures in the last test.

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The first two failures are caused by the bug assuming that the Proxy trap receiver argument may be equal to the target.

The other two failures are because WebIDL doesn't exclude array index properties from named properties when an object doesn't support indexed properties, which WebIDL2JS does anyway.

[TimothyGu]

Looks good. I believe this is a relatively recent change in heycam/webidl#059491b4.

[ExE-Boss]

@TimothyGu The # symbol refers to an in issue, to refer to a commit, you have to use the whatwg/webidl@059491b syntax.

Also, array index properties weren’t excluded before whatwg/webidl@059491b either.