The default value of this property, when it has not been set to a specific value.
For most DiagnosticsProperty classes, if the value of the property equals defaultValue, then the priority level of the property is downgraded to DiagnosticLevel.fine on the basis that the property value is uninteresting. This is implemented by isInteresting.
The defaultValue is kNoDefaultValue by default. Otherwise it must be of
final Object? defaultValue;