* Use specific event type for nativeEvent property
Allows event handlers to fully use the `ev.nativeEvent` property without casting. But it does make the `React.MouseEvent` type slightly more complicated.
* Maintain backwards compatibility of SyntheticEvent type
TypeScript 2.3 should be adding type parameter defaults, which allows the `nativeEvent` property to be made generic but without breaking existing code.
* Override nativeEvent in the more specific event types
Overriding nativeEvent in this way maintains backwards compatibility and doesn't depend on bleeding-edge TypeScript features.
