double lerpDouble(num a, num b, double t)

Linearly interpolate between two numbers.

Source 

double lerpDouble(num a, num b, double t) {
  if (a == null && b == null)
    return null;
  if (a == null)
    a = 0.0;
  if (b == null)
    b = 0.0;
  return a + (b - a) * t;
}