I don't understand the purpose of `dfs`.  It is empty and never assigned to.  Given that this is an `@NgFilter` class it will only be instantiated by the dependency injector and access to this instance will be limited to use via an AngularDart pipe.  Hence, clients will not have access to `dfs`.

If I were to guess a use for `dfs` it would be as a cache. In which case it should be populated with values just before line 61 after a value is obtained for `df`; i.e.,

    dfs[format] = df;

In this case `dfs` should be made private: `_dfs`.