Fixes#5038
The problem with `constructor` is that it doesn't have `context` yet when being called. It's also considered unsafe to add a side-effect on constructor except when server-rendering
* Layout ground works for next/async
* Implement the Dynamic Bundle feature.
* Add some test cases.
* Update README.
* Implement props aware dynamic bundle API.
* Update tests and README.
* Add a test case for React Context support.
* Always check with the fs when gettings chunks.
* Add a new set of test cases for dynamic imports in dev.
* Add dynamic import test cases for production.
* Add availableChunks support for static exports.