Add a rule prefer-covariant-props

[julienw]

With react and flow, it's very common to type props like this:

type Props = {
  prop1: string,
  prop2: number
};

However this makes it possible to write to the props object, like this: this.props.prop1 = 'foo' which is to be avoided.
To force this with flow, the solution is to use covariant properties:

type Props = {
  +prop1: string,
  +prop2: number
};

It would be useful to have an eslint rule to force this pattern.

[ljharb]

I'm a bit hesitant to have Flow-only rules in a non-flow-specific eslint plugin, but I agree that such a rule would be a good thing to have if using Flow.

[julienw]

Yeah I hesitated between filing this or in eslint-plugin-flowtype, but I thought it was more react-specific than flow-specific :)

[EvHaus]

Closing this as a duplicate of #1869

[julienw]

👍