In a Dart program, I tried to create a custom ErrorListener to capture SyntaxError() for the parser and lexer. But, I couldn't because the compiler said that Recognizer is unknown. It turns out that it is not in the list of classes exported. Recognizer is the first parameter in SyntaxError. I also see Interval and IntervalSet are not exported as well. You can see what is exported here. These classes are all essential in programming with Antlr.