Per language team discussion (summarized briefly here we are changing the interpretation of a pubspec sdk constraint with a missing lower bound (or in general no sdk constraint at all, though that will become a pub error) to imply a language version of 2.7. I believe that the analyzer is one of the few tools that reads pubspec.yaml directly, and hence this will need to be changed there.

This is blocker for beta, and will need to be cherry picked if it doesn't make the beta branch cut.

cc @scheglov @devoncarew

cc @pcsosinski @mit-mit @franklinyow