Type spread over union type doesn't work

[EXio4]

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

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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);

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What is the expected behaviour?

To be consistent with Flow's semantics

Try Flow example

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Which package versions are you using?

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