Checks whether the file system entity with this path exists. Returns
Future<bool> that completes with the result.
Since FileSystemEntity is abstract, every FileSystemEntity object is actually an instance of one of the subclasses File, Directory, and Link. Calling exists on an instance of one of these subclasses checks whether the object exists in the file system object exists and is of the correct type (file, directory, or link). To check whether a path points to an object on the file system, regardless of the object's type, use the type static method.