double opacity

The fraction to scale the child's alpha value.

An opacity of 1.0 is fully opaque. An opacity of 0.0 is fully transparent (i.e., invisible).

The opacity must not be null.

Values 1.0 and 0.0 are painted with a fast path. Other values require painting the child into an intermediate buffer, which is expensive.

Source 

double get opacity => _opacity;
void opacity=(double newOpacity)

Source 

set opacity (double newOpacity) {
  assert(newOpacity != null);
  assert(newOpacity >= 0.0 && newOpacity <= 1.0);
  if (_opacity == newOpacity)
    return;
  _opacity = newOpacity;
  _alpha = _getAlphaFromOpacity(_opacity);
  markNeedsCompositingBitsUpdate();
  markNeedsPaint();
  markNeedsSemanticsUpdate();
}