Made some delegate methods optional in TreeViewDelegate and ListViewDelegate. Fixed #402: Keep expanded/collapsed state of tree nodes.
rename chrome_gen import prefixes to chrome
Simplified code
Merge pull request #429 from dinhviethoa/402-keep-expanded-state

Keep collapsed/expanded state of the files view
Merge pull request #432 from dart-lang/chrome_gen

rename chrome_gen import prefixes to chrome
Merged with upstream