lerp method Null safety
- double t
override
    Returns the value this variable has at the given animation clock value.
The default implementation of this method uses the +, -, and *
operators on T. The begin and end properties must therefore be
non-null by the time this method is called.
In general, however, it is possible for this to return null, especially
when t=0.0 and begin is null, or t=1.0 and end is null.
Implementation
@override
int lerp(double t) => (begin! + (end! - begin!) * t).round();