Release process: Create a test for validating webdev, dwds in preparation for SDK releases

[annagrin]

Create a test that will validate that an upcoming SDK release works with webdev.

We currently run local webdev in webdev tests. For a smooth release process it is important to validate that the latest global version of webdev works with all the the other latests packages.

Proposal:

• Keep webdev tests always using local webdev and whatever version of dwds it works with currently.
• Create a top level project _package_tests that run e2e tests (or more) on fixtures that use latest published versions of webdev and dwds.
  • run dart pub upgrade on the fixtures and verify that pubspec contains latest versions for build_web_compilers, build_runner, dwds, etc.
  • run those tests for the latest stable, beta, and dev SDK in a cron task. (Ideally we would like to run them after publishing a version of webdev)