Update .gitignore to new `dart_tool` pub cache (#60)

* Update .gitignore to new `dart_tool` pub cache

dart-lang/sdk#32030

* delete .pub and packages

https://www.dartlang.org/guides/libraries/private-files
diff --git a/.gitignore b/.gitignore
index 7a2e8dd..e50bf96 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,9 +1,11 @@
+# See https://www.dartlang.org/guides/libraries/private-files
+
 .buildlog
 .DS_Store
 .idea
-.pub/
+
+.dart_tool/
 .settings/
 build/
-packages
 pubspec.lock
 .packages