debugAssertIsValid method
- @override
override
    In checked mode, throws an exception if the object is not in a valid configuration. Otherwise, returns true.
This is intended to be used as follows:
  assert(myDecoration.debugAssertIsValid());
Implementation
@override
bool debugAssertIsValid() {
  assert(lightColor != null
      && darkColor != null
      && textColor != null
      && style != null
      && margin != null
      && _position != null
      && _position.isFinite
      && _opacity != null
      && _opacity >= 0.0
      && _opacity <= 1.0);
  return true;
}