[Dart] minor adjustments to support stricter dart analyzer configurations.

Edit: Dart added support for disabling all lints at once:

 // ignore_for_file: type=lint

This should now be preferred over enumerating all to-be-ignored lints explicitly.

(The original issue description:)

• It would be great if the following rules could also be excluded here:

https://github.com/antlr/antlr4/blob/09e917ea8b0939c835c38189031c93840f3c41cc/tool/resources/org/antlr/v4/tool/templates/codegen/Dart/Dart.stg#L186

// ignore_for_file: unused_import, unused_local_variable, prefer_single_quotes, file_names, directives_ordering, prefer_final_locals, avoid_catching_errors, unnecessary_parenthesis, prefer_const_declarations

file_names: Dart prefers snake_case over PascalCase. directives_ordering: generated directives at the top of the file are not ordered. prefer_final_locals: mutable local values should be marked as final. avoid_catching_errors: the errors within antlr should extend the Exception type. unnecessary_parenthesis: self explanatory. prefer_const_declarations: self explanatory.

• The following line should not have dynamic as its type, but use var to allow the compiler to infer the type. The dart analyzer doesn't support this type of 'dynamic' use when some stricter checks are enabled.

https://github.com/antlr/antlr4/blob/09e917ea8b0939c835c38189031c93840f3c41cc/tool/resources/org/antlr/v4/tool/templates/codegen/Dart/Dart.stg#L321

var _localctx = ...