* 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.
4fcc25a229