IconThemeData fallback({Color color: const Color(0xFF000000), double opacity: 1.0, double size: 24.0 })

Creates an icon theme that is identical to this icon theme but with any null fields filled in. Specific fallbacks can be given, but in their absence, this method defaults to black, fully opaque, and size 24.0.

Source 

IconThemeData fallback({
  Color color: const Color(0xFF000000),
  double opacity: 1.0,
  double size: 24.0
}) {
  if (this.color != null && this.opacity != null && this.size != null)
    return this;
  return new IconThemeData(
    color: this.color ?? color,
    opacity: this.opacity ?? opacity,
    size: this.size ?? size
  );
}