LGTM, thanks!
https://codereview.appspot.com/583320043/diff/581420044/flower/file-name.cc File flower/file-name.cc (right): https://codereview.appspot.com/583320043/diff/581420044/flower/file-name.cc#newcode124 flower/file-name.cc:124: s += ext_; any reason to use two lines? In other files you use a single line for similar things. https://codereview.appspot.com/583320043/
