Defines the tokens that are produced by the scanner, used by the parser, and referenced from the AST structure.
- The keywords in the Dart programming language.
- A specialized comment token representing a language version (e.g. '// @dart = 2.1').
- A token that was scanned from the input. Each token knows which tokens precede and follow it, acting as a link in a doubly linked list of tokens.
- The types of tokens that can be returned by the scanner.