Previously `ExtensionAssetGetter` subclassed `DependencyFileGetter`,
commit 6e7087ee88b34af453e20f0d8dda95438c72a8fb changed its
functionality to use a cached location instead of the local resource
location. This commit changes the `ExtensionAssetGetter` to subclass
`EnvironmentDependencyGetter` so it checks the local resource location
again.