Fix tests for latest pub parsing rules for environment (dart-lang/pubspec_parse#60)

Since https://github.com/dart-lang/pub/commit/656803e924eae5cf6574f5200232ef69ac3b4f92  (SDK-2.12.0-28.0.dev) pub requires an 
SDK constraint.

Update tests so this requirement no longer causes failures.
Also updated test expectations for latest `pkg:json_annotation` changes