Update Dart.gitignore

add the `.pub/` directory, and change the `packages` filter to match sub-folders.

Dart.gitignore

@@ -1,7 +1,8 @@
 # Don’t commit the following directories created by pub.
-build/
-packages/
 .buildlog
+.pub/
+build/
+packages
 
 # Or the files created by dart2js.
 *.dart.js