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c228924543
* Index signatures contribute properties to unions
This means that in a union like this:
```ts
type T = { foo: number } | { [s: string]: string }
```
`foo` is now a property of `T` with type `number | string`. Previously
it was not.
Two points of interest:
1. A readonly index signature makes the resulting union property readonly.
2. A numeric index signature only contributes number-named properties.
Fixes #21141
* Correctly handle numeric and symbol property names
1. Symbol-named properties don't contribute to unions.
2. Number-named properties should use the numeric index signature type,
if present, and fall back to the string index signature type, not the
other way round.