Update .gitignore to new dart_tool pub cache

pq · pq · commit b9995f94dedc · 2018-03-10T14:43:00.000-08:00
See: dart-lang/sdk#32030
diff --git a/.gitignore b/.gitignore
@@ -1,5 +1,5 @@
 .packages
-.pub
+.dart_tool
 packages
 pubspec.lock
 doc/api/
