evict method
- String key
override
    If this is a caching asset bundle, and the given key describes a cached asset, then evict the asset from the cache so that the next time it is loaded, the cache will be reread from the asset bundle.
Implementation
@override
void evict(String key) {
  _stringCache.remove(key);
  _structuredDataCache.remove(key);
  _structuredBinaryDataCache.remove(key);
}