num alpha

The alpha transparency value of the object specified.

Valid values are 0 (fully transparent) to 1 (fully opaque). The default value is 1. Display objects with alpha set to 0 are active, even though they are invisible.

Source 

@override
num get alpha => _alpha;
void alpha=(num value)

Source 

@override
set alpha(num value) {
  if (value is num) {
    // Clamp values and convert possible integers to double.
    if (value <= 0) value = 0.0;
    if (value >= 1) value = 1.0;
    _alpha = value;
  }
}