PluginsRegistryFile constructor
Creates a PluginsRegistryFile bound to the given VirtualFs and
projectRoot.
@param _fs File-system abstraction (production: RealFs;
tests: InMemoryFs).
@param _projectRoot Absolute path to the Flutter project root. The
registry lives at
<_projectRoot>/.artisan/plugins.json.
Implementation
PluginsRegistryFile(this._fs, this._projectRoot);