A WIP destination for golden file testing.

See b/147830720.

TIP: Need to update all of the tests quickly? Use:

$ third_party/dart_src/angular/goldens/update_all.sh

Currently when adding a new a.dart file, you must add a sibling a.template.dart.golden and a.js.golden file, otherwise the resulting golden_test macro will fail saying those files are not found.