Fix a class variable initialization bug in treediff.
Merge pull request #3331 from dart-lang/fix_bug_in_treediff

Fix a class variable initialization bug in treediff.
rename app/ to web/; refactoring ensues
fix bots
fix the mode-test step
Merge branch 'master' into devoncarew_app_to_web
Merge pull request #3332 from devoncarew/devoncarew_app_to_web

rename app/ to web/; refactoring ensues
Merge remote-tracking branch 'upstream/master' into small-fixes-3