Release breaking changes 0.33.0 and 0.34.0

[srawlins]

I have a few breaking changes which have landed, and a few pending, and I'd like to make smaller releases (and bumps), so these will be (at minimum) 0.33.0 and 0.34.0. @devoncarew also points out that #2337 needs to be carefully merged into the SDK, so I'm documenting the process for the next few releases:

1. Import b039e21 internally (this was in-progress before remove a use of resource loading #2337 landed).
2. Bump the Dart SDK to b039e21.
3. Import ab98003 (remove a use of resource loading #2337) internally.
4. Bump the Dart SDK to ab98003, coordinated with @devoncarew .
5. Cut 0.33.0 at ab98003.
6. Land new breaking changes like PackageConfigProvider, MockSdk, etc for improved unit testing #2332.
7. Import PackageConfigProvider, MockSdk, etc for improved unit testing #2332 internally.
8. Bump the Dart SDK including PackageConfigProvider, MockSdk, etc for improved unit testing #2332.
9. Cut 0.34.0 at the commit imported internally and in the Dart SDK.