'Fetch all sources that we know are required (being referenced directly
in frames).'

def populate_source_cache(self, frames):

    pending_file_list = set()
    for f in frames:
        if (f.get('abs_path') is None):
            continue
        if (f['abs_path'] == '<anonymous>'):
            continue
        pending_file_list.add(f['abs_path'])
    for (idx, filename) in enumerate(pending_file_list):
        self.cache_source(filename=filename)
