It looks like this formatter isn't just for internal use, at a company that can buy each of their developers multiple 25-inch monitors. Not everyone has screen space to burn making their editors and terminals arbitrarily wide. If the majority of Dart code was formatted with a line length limit above 80, that would significantly deter me from exploring Dart.
I don't contribute to many open-source projects. But I'd imagine Python would have a good track record, PEP8 specifies something like a 72-character limit.
That said, long lines in an existing project aren't a dealbreaker, if I have other reasons for contributing. (The one I'm working on now has them.) But if there's no reasonable way for me to write code in a language without breaking 80 chars, I'm not likely to choose that language for my own projects.
