Dart tests fail

[parrt]

@lingyv-li It looks like some of the tests are failing somehow now in DART target:

https://ci.appveyor.com/project/parrt/antlr4/builds/34724730

Failed tests: 
  TestCompositeLexers>BaseRuntimeTest.testOne:123->BaseRuntimeTest.testLexer:200->BaseRuntimeTest.assertCorrectOutput:363 [Dart:LexerDelegatorRuleOverridesDelegate] Parse output is incorrect: expectedOutput:<M.A
[@0,0:1='ab',<1>,1:0]
[@1,2:1='<EOF>',<-1>,1:2]
>; actualOutput:<>; expectedParseErrors:<>; actualParseErrors:<M.dart:30:7: Error: A value of type 'Null' can't be assigned to a variable of type 'String'.
      null, null, null, "'b'"
      ^
M.dart:30:13: Error: A value of type 'Null' can't be assigned to a variable of type 'String'.
      null, null, null, "'b'"
            ^
M.dart:30:19: Error: A value of type 'Null' can't be assigned to a variable of type 'String'.
      null, null, null, "'b'"
                  ^
M.dart:33:7: Error: A value of type 'Null' can't be assigned to a variable of type 'String'.
      null, "A", "WS", "B"
      ^
>; expectedToolErrors:<>; actualToolErrors:<>.

[lingyv-li]

I'll look into this today.