Type spread over union type doesn't work

This is a:

- [x] Bug Report
- [ ] Feature Request
- [ ] Question
- [ ] Other

Which concerns:

- [x] flow-runtime
- [x] babel-plugin-flow-runtime
- [ ] flow-runtime-validators
- [ ] flow-runtime-mobx
- [ ] flow-config-parser
- [ ] The documentation website

---

### What is the current behaviour?

A type spread over a union creates invalid code

```
type C1 = { type: "C1", test1: string };

type C2 = { type: "C2", test2: number }

type Cs = C1 | C2

type Err = { id: number, ...Cs }

console.log(Err.properties);
```
---

### What is the expected behaviour?

To be consistent with Flow's semantics

[Try Flow example](https://flow.org/try/#0C4TwDgpgBAwgjFAvFA3lUkBcUBE8cA06EAzsHNmQE4CWAdgOZQC+A3AFDsbQwBMSqdOAjY8vQsTK9sdAK4BbAEYQqLTt1gkB8KAB9YvdcKgBRKquRoaAExkLlVIgDoXMLc04BjAPZ0yUaGwzC0EbbDgAdiINUXxo0nJRAHdvKgBregYcFiA)

---

### Which package versions are you using?

Reproducible on live demo https://codemix.github.io/flow-runtime/#/try