def DiscoverClasses(start_dir, top_level_dir, base_class, pattern='*',
index_by_class_name=False):
"""Discover all classes in |start_dir| which subclass |base_class|.
def DiscoverClasses(start_dir, top_level_dir, base_class, pattern='*',
index_by_class_name=False):
"""Discover all classes in |start_dir| which subclass |base_class|.