2.12.0-31.0.dev causes dartdoc test failures due to missing lower constraints in test packages

[jcollins-g]

This is cloaked by #2422.

dart2-dartdoc_test.dart: 01:24 +6 ~1 -7: dartdoc with generators generate docs with custom templates [E]
dart2-dartdoc_test.dart:
dart2-dartdoc_test.dart:   pub get failed: pubspec.yaml has no lower-bound SDK constraint.
dart2-dartdoc_test.dart:   You should edit pubspec.yaml to contain an SDK constraint:
dart2-dartdoc_test.dart:
dart2-dartdoc_test.dart:   environment:
dart2-dartdoc_test.dart:     sdk: '>=2.10.0 <3.0.0'
dart2-dartdoc_test.dart:
dart2-dartdoc_test.dart:   See https://dart.dev/go/sdk-constraint