build_config errors test text has changed on 3.0 dev

[jakemac53]

The test is test/errors_test.dart: for null exclude globs, error looks like below. Skipping for now, ideally we just fix the message and unskip.

 ACTUAL
  r'''
  line 7, column 11 of build.yaml: Unsupported value for "exclude". type 'Null' is not a subtype of type 'String' of ' in type cast'
    ╷
  7 │           -
    │           ^
    ╵'''
  Expected: throws <Instance of 'ArgumentError'> with `message`: 'line 7, column 11 of build.yaml: Unsupported value for "exclude". type \'Null\' is not a subtype of type \'String\' in type cast\n'
              '  ╷\n'
              '7 │           -\n'
              '  │           ^\n'
              '  ╵'
    Actual: <Closure: () => BuildConfig>
     Which: threw ArgumentError:<Invalid argument(s): line 7, column 11 of build.yaml: Unsupported value for "exclude". type 'Null' is not a subtype of type 'String' of ' in type cast'
                    ╷
                  7 │           -
                    │           ^
                    ╵>

[jakemac53]

Filed dart-lang/sdk#50756