analyzer: tell me if package-root contains something I'm not using

[kevmoo]

From @kevmoo on March 7, 2013 0:47

Does library W import library X, but never uses code from X? Does package Y define package Z as a dependency, but never imports from Z?

Great things for analyzer to tell me

Copied from original issue: dart-lang/sdk#8967

[kevmoo]

From @iposva-google on March 7, 2013 7:15

Added Area-Analyzer, Triaged labels.

[kevmoo]

From @bwilkerson on March 7, 2013 16:1

Added this to the Later milestone. Removed Type-Defect label. Added Type-Enhancement label.

[kevmoo]

From @bwilkerson on April 1, 2014 16:29

Added Analyzer-Hint label.

[kevmoo]

From @kasperl on July 10, 2014 10:20

Removed this from the Later milestone. Added Oldschool-Milestone-Later label.

[kevmoo]

From @kasperl on August 4, 2014 7:43

Removed Oldschool-Milestone-Later label.

[srawlins]

Since we have the unused_imports hint now, I've adjusted the title to just reference the "Unused package dependency" bit. Not sure if this is feasible though...