Add `.dart_tool/`

Followup to https://github.com/dart-lang/site-www/issues/444#issuecomment-346560788

Dart.gitignore

@@ -1,6 +1,7 @@
 # See https://www.dartlang.org/tools/private-files.html
 
 # Files and directories created by pub
+.dart_tool/
 .packages
 .pub/
 build/