Update test to allow type variable type literals as potential constants

[johnniwinther]

The tests

language/constants_2018/constant_types_test/04 Pass (expected CompileTimeError)

needs to be updated when dart-lang/language#1860 has landed

[eernstg]

Cf. https://dart-review.googlesource.com/c/sdk/+/214644.

[eernstg]

Said tests landed as 117d1b7.