assets.py

class DebugLevelFilter(Filter):
    max_debug_level = None

    def __init__(self, required_level):
        super(DebugLevelFilter, self).__init__()
        self.required_level = required_level

    def input(self, in_, out, source_path, **kw):
        if self.required_level == self.env.debug:
            out.write(in_.read())
        else:
            skip_msg = '/* Skipped {} because of debug level */'.format(os.path.basename(source_path))
            out.write(skip_msg)


jquery = Bundle('js/jquery/underscore.js',
                'js/jquery/jquery.js',
                Bundle('js/jquery/jquery-migrate-silencer.js', filters=DebugLevelFilter(required_level=False),
                       output='jquery_migrate_silencer_%(version)s.js'),
                'js/jquery/jquery-migrate.js',
                ...)