defaultToStringOver method
This instance's own default rendering when native — its bridged
super-object or proxy — answers Object's default toString, and null
when native overrides it (ArgumentError's message is a real answer).
The one place the rule lives: a member read, super.toString() and
interpolation all ask it (SCF39).
Implementation
String? defaultToStringOver(Object native) =>
_isObjectDefaultToString(native) ? _diagnosticString : null;