ace_test.js : invert the order in the loader script to try first the `ace_test.dart.js`

Chrome 33 seems to be okay with Dart scripts so we no longer get an
error ... this changes to trying to load first the js; it won't be there
until we `pub build` so we'll get an error and fall back to the dart
script.  It means we get an error in development but it can safely be
ignored ...