dillon1000/react

Support throwing null (#10213)

* Support throwing null

In JavaScript, you can throw values of any type, not just errors. That
includes null. We currently rely on null checks to determine if a user-
provided function has thrown. This refactors our error handling code to
keep track of an explicit boolean flag instead.

* Add DOM fixture test case for break on exception behavior

* preventDefault error events during feature test

We call invokeGuardedCallbackDev at startup as part of a feature test.
But we don't want those errors to log to the console.

* Add throwing null test case

* Use ReactFeatureFlags instead of ReactDOMFeatureFlags

React ART uses this, too.

* Non-errors in error logger

If a non-error is thrown, we'll coerce the value to a string and use
that as the message.