Update .gitignore to new dart_tool pub cache (#40)

See dart-lang/sdk#32030

Author: bcko

.gitignore

@@ -1,15 +1,6 @@
+# See https://www.dartlang.org/guides/libraries/private-files
 # Don’t commit the following directories created by pub.
-.buildlog
-.pub/
-build/
-packages
+.dart_tool/
 .packages
-
-# Or the files created by dart2js.
-*.dart.js
-*.js_
-*.js.deps
-*.js.map
-
-# Include when developing application packages.
-pubspec.lock
+.pub/
+pubspec.lock