I’ve asked a friend about this on Twitter (who is a Google Developer Expert for Flutter) and this is what he had to say:

Inheriting mistakes from other languages is so common. Null is a classic example of languages inheriting the mistake all the way back from Algol that even the creator of it admitted was his biggest mistake.

I never used Dart, but I dislike inconsistency, so I’d hate typing String instead of string if everything else is lowercase.