Widen version dependencies for `package:angular`, `package:code_transformers`,
and `package:observe`.
`package:guinness` uses `package:unittest` while
the newest versions of `package:code_transformers` use `package:test`.
This causes our end-to-end Dart transformer tests (which use testing
code in `package:code_transformers`) to be skipped.
To fix this:
- Move e2e tests to run in a separate file
- Run `gulp test.server.dart` tests serially
Closes#5922Closes#5935