unListen method
Removes registered listener from connection. See also [listen!].
Implementation
unListen(JsRef conn, String key) {
// FIXME: the listener should be removed properly.
context.runtime.onMessage(key, (report) {});
var code = """
vendor.ds.unlisten(${conn.toJsCode()}, '$key');
""";
return context.evaluate(code);
}