* Simplify path handling logic in dependency checker and devFS
Simplification will make it easier to port this to Windows.
* Roll Engine to 0a7b177c330367904597a6129b3eb653d29dfca0
- [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