Analyzer: Refactor _PSDependency factory constructor to not return null
Analyzer: Refactor _PSDependency factory constructor to not return null

Change-Id: I8bab14bacc14c88fc0d0c3276aee1d48be40116a
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/162101
Reviewed-by: Brian Wilkerson <1f7641b6b14c52b9163524ab8d9aabff80176f21@google.com>
Reviewed-by: Phil Quitslund <3faecbdd7b7be83f493f037bf1d597e6bb9d0848@google.com>
Commit-Queue: Samuel Rawlins <5cf2cac5eec2ad6d0eb2b0f4ff6af9a590ce2bf6@google.com>

dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk