gistfile1.txt

// Solved by re-creating the outer maps so the inner maps could be cast.

  final results = new List< Map<Object, Map<Object, num> > >();
    for (final value in windowValues)
    {
       Map<Object, Map<Object, num> > outerMap =
         new Map<Object, Map<Object, num> >.fromEntries(
           (value as Map).entries.map( (entry) => new MapEntry<Object, Map<Object, num> >(entry.key, (entry.value as Map).cast<Object, num>() ) )
         );

       results.add(outerMap);
    }