Update .gitignore to new dart_tool pub cache (flutter#14)

dart-lang/sdk#32030

Author: bcko

.gitignore

@@ -1,15 +1,5 @@
 # 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