putIfAbsent method
If isImmutable is false, or the key already exists, then allow the edit. Throw JsonObjectLiteException if we're not allowed to add a new key
Implementation
@override
void putIfAbsent(dynamic key, Function() ifAbsent) {
if (isImmutable == false || containsKey(key)) {
_objectData.putIfAbsent(key, ifAbsent);
} else {
throw const JsonObjectLiteException('JsonObject is not extendable');
}
}