toString method
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 StringBuffer buffer = StringBuffer('MirekException');
  if (message != null) {
    buffer.write(': $message');
  }
  if (value != null) {
    buffer.write('\n'
        '\tExpected: a number between 153 and 500 (inclusive)\n'
        '\tActual: $value');
  }
  return buffer.toString();
}