* Add a check in case the flutter directory in .packages no longer exists. Clean up and prompt user
* Update documentation to use flutter packages get for end-users instead of flutter update-packages.
* Merge missing sdk error with the multiple sdk error. They're really the same thing.
* Use flutterPath in both checks.
* Change file_system’s copy folder to copy director which takes into account the file system
* Test support files
* Add test and split into 2 messages again.
* Move tests to run in memory file system's copy. Tested with dev/bots/test.sh
- [x] Introduce DependencyChecker which can determine if any dependencies have been modified.
- [x] Move the DartDependencyBuilder into a separate file.
- [x] Add unit tests for DartDependencyBuilder.
- [x] Add unit tets for DependencyChecker
Part of #7014
