Add git ignore for .dart_tool (#1624)

* Add git ignore for .dart_tool

* Sort .gitignore so it is more clear it covers everything

Author: jcollins-g

.gitignore

@@ -1,21 +1,21 @@
-build/
-doc/
-testing/test_package/doc
-testing/test_package_small/doc
-packages
 .buildlog
-.pub
-
-.idea
+.DS_Store
 .atom/
-
+.dart_tool/
+.idea/
 .packages
+.pub
 .settings/
-.DS_Store
-*.iml
-
+build/
+doc/
+packages
 pub.dartlang.org/
+testing/test_package/doc
+testing/test_package_small/doc
 
 *.dart.js
+*.iml
+*.ipr
+*.iws
 *.js.deps
 *.js.map