Add tests for BaseAsset.
Update AssetCollector::setDefaultMetadata to derive type from the bag.
Remove unnecessary babysitting from AssetLibraryRepository.
Bring AssetCollection up to what will be 100% coverage.
More converting of bag -> collection in AssetCollectorTest.
Add lastCss after() behavior to collectors; bring them to 100% coverage.
Some tweaks to ExternalAsset and StringAsset.
Incremental changes to new css grouper - but not done yet.