fix: serialize thenables in hydratable

[Ocean-OS]

This serializes thenables in hydratable. However, there is a slight discrepancy: if you have an object that is a thenable on the server, it will be converted to a Promise on the client. I'm not sure if we should:

1. convert thenables to Promises (and maybe reflect that in the types?)
2. do nothing and close this PR
3. resolve the thenable on the server, and on the client return an object that has the serialized contents of the thenable (excluding the original then function), with a then function that resolves to the awaited value, which would look like:

const create_thenable = () => ({
  foo: 1,
  bar: 2,
  then(onresolve, onreject) {
    return Promise.resolve(Math.random()).then(onresolve, onreject);
  }
});
const thenable = await hydratable('test', create_thenable);
// on client: Object.assign({ foo: 1, bar: 2 }, { then: (a, b) => r(0.1234).then(a, b) });

Option 3 would probably be the best in terms of correctness, but it'd also be the most complex; we'd have to add some extra errors and checks for values that cannot be serialized (since there isn't a way to omit a property from a non-POJO while preserving its prototype, which would let us make devalue do the checking for us), which would raise the issue of having two types of serialization errors: those from devalue and those from svelte, which would make error handling trickier. Additionally, we'd likely have to preserve key order in thenables, which could get ugly.

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Playground

pnpm add https://pkg.pr.new/svelte@17275

[Rich-Harris]

Is this related to #17235? My feeling is that we should handle cross-realm promises, but otherwise ignore anything funky — serializing non-promise thenables feels no different than serializing any other non-built-in type

[Ocean-OS]

I hadn't seen #17235 before, but I do agree with you. Is there a way to check if a value is a cross-realm Promise other than Object.prototype.toString, or is that the best we could do?

[Rich-Harris]

Not that I'm aware of. But the toString check is in line with what devalue does for other things (Date, Set, etc) so I think it's fine. Just merged #17284

[Ocean-OS]

Closing in favor of #17284