`instanceof builtin` checks fail for proxies using `getPrototypeOf` trap

[Rich-Harris]

What version of Bun is running?

1.3.1+89fa0f343

What platform is your computer?

Darwin 24.6.0 arm64 arm

What steps can reproduce the bug?

bun run the following:

class Foo {}

const proxy = new Proxy(new Foo(), {
	getPrototypeOf() {
		return File.prototype;
	}
});

console.log('proxy instanceof Foo', proxy instanceof Foo);
console.log('proxy instanceof File', proxy instanceof File);

What is the expected behavior?

It should log false then true, as happens in Node and other runtimes.

What do you see instead?

Instead, it logs false then false. It behaves correctly if instead of File you use some non-built-in class.

Additional information

ref sveltejs/kit#15018