event (lowercased) is not being type and declaration checked

Hey guys, and small bug here:

If you use a variable, named `event`, even if it was never declared in your scope, flow does not throw an error about it.

example:
```
/@flow
// No declaration of any kind
const work = `${event.toString()}`; // Should fail
const fail = `${evnt.toString()}`; // Fail as expected
```
https://flowtype.org/try/#0PQKgBAAgZgNg9gdzCYAoVBjOA7AzgFzATgCcBrMAXjAAMASAbwFMA3J7fAOnzgGV8SAS2wBzABQBKAL40A3JhwEwUAIaCYVWo1YdufAcPHS5QA

I believe it is related to this:
https://github.com/facebook/flow/blob/v0.40.0/lib/dom.js#L3090

What do you think? Thank you team in advance!