toString method
override
    A string representation of this object.
Some classes have a default textual representation,
often paired with a static parse function (like int.parse).
These classes will provide the textual representation as
their string representation.
Other classes have no meaningful textual representation
that a program will care about.
Such classes will typically override toString to provide
useful information when inspecting the object,
mainly for debugging or logging.
Implementation
@override
String toString() {
  final List<String> description = <String>[
    'center: $center',
    'radius: ${debugFormatDouble(radius)}',
    'colors: $colors',
    if (stops != null) 'stops: $stops',
    'tileMode: $tileMode',
    if (focal != null) 'focal: $focal',
    'focalRadius: ${debugFormatDouble(focalRadius)}',
    if (transform != null) 'transform: $transform',
  ];
  return '${objectRuntimeType(this, 'RadialGradient')}(${description.join(', ')})';
}