Attempt to solve Bug #2795. I've added a button in the UI and implemented added the backend function that changes the rootDirectory
suggested modification implemented but still without the redundancy on _showRootDirectory
Fixed the problem with the code duplication on showRootDirectory. Other style issues also solved
added the showRootDirectory stub intro the SparkModel
Merge pull request #2930 from dart-lang/mywork

Attempt to solve Bug #2795. review @dinhviethoa