Support context components in react-test-renderer

[pshrmn]

Potential fix for #12150. I had to use the pendingProps instead of memoizedProps for the ContextConsumer (I don't know enough about React's internals to know why that doesn't have a memoizedProps).

[gaearon]

I think we shouldn't be using pendingProps. Maybe type.value is enough there.

That I said I'm not convinced that context providers and consumers need to be in the output at all. What's the point of asserting on them if they don't contain their own logic?

I think maybe we should just skip over them.

[pshrmn]

If I understand you correctly, ContextConsumer and ContextProvider should be treated somewhat like a Fragment? I see two places where Fragments are used here:

1. In toTree they just pass on their child Fiber's tree instead of returning themselves.
2. When getting the children of an element (for querying with the find___ methods), the Fragment will skip to its child. This means that the find___ methods will never match a Fragment.

I think the first definitely makes sense to replicate with the ContextConsumer and ContextProvider. The second probably makes sense, but I'm not as confident there.

There is also one issue that I noticed. While using the children getter works for ignoring nested elements, the root will still be matched by the find___ methods. This isn't an issue with Fragments because reconcileChildFibers bypasses a Fragment.

// if root.children does not return ContextConsumers and ContextProviders
const { Provider, Consumer } = React.createContext('waldo');

// this works
const inst1 = renderer.create(
  <Provider value='waldo'>
  </Provider>
);
const p1 = inst.root.findByType(Provider);

// this throws
const inst2 = renderer.create(
  <div>
    <Provider value='waldo'>
    </Provider>
  </div>
);
const p2 = inst.root.findByType(Provider);

[pshrmn]

I rebased after #12154, but I'm still not positive exactly what the behavior should be, and since I know you want to get 16.3.0 out soon, it might make more sense for someone on the React team to make these changes instead.

[pshrmn]

#12237

[acdlite]

@pshrmn Shoot, I didn't see this PR before opening #12237. I'm sorry :(

[pshrmn]

@acdlite No worries, just glad to see a solution. 😄