Fixed the crash on initializing "#renameDialog" in Polymer UI

The reason was: Polymer didn't like identical element IDs for the file name input control ("#fileName") under two different dialogs, "#fileNewDialog" and "#renameDialog". dialog.querySelector("#fileName") returned the correct element for the former, but null for the latter. The same worked fine in non-Polymer version.
Merge pull request #470 from dart-lang/fix-polymer-crash-on-rename-file-dialog

Fixed the crash on initializing "#renameDialog" in Polymer UI