[Dart] Fix for #4320--export additional types #4340
                
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This PR fixes #4320.
It is required for porting grammars-v4/python/python3 to Go. I am blocked without this change.
Sometimes additional tokens must be created and emitted in the token stream by the lexer. The API to do that is in most targets--except Dart. This change fixes that, exporting Pair and MultiMap used in CommonToken creation.