Base class for Flutter Driver wait conditions, objects that describe conditions the driver can wait for.
This class is sent from the driver script running on the host to the driver
extension on device to perform waiting on a given condition. In the extension,
it will be converted to a
WaitCondition that actually defines the wait logic.
If you subclass this, you also need to implement a
WaitCondition in the extension.
A const constructor to allow subclasses to be const.