dillon1000/react

Lazy components must use React.lazy (#13885)

Removes support for using arbitrary promises as the type of a React
element. Instead, promises must be wrapped in React.lazy. This gives us
flexibility later if we need to change the protocol.

The reason is that promises do not provide a way to call their
constructor multiple times. For example:

const promiseForA = new Promise(resolve => {
  fetchA(a => resolve(a));
});

Given a reference to `promiseForA`, there's no way to call `fetchA`
again. Calling `then` on the promise doesn't run the constructor again;
it only attaches another listener.

In the future we will likely introduce an API like `React.eager` that
is similar to `lazy` but eagerly calls the constructor. That gives us
the ability to call the constructor multiple times. E.g. to increase
the priority, or to retry if the first operation failed.