Method not yet implemented error while using javascriptExecutor while trying to access polymer DOM elements

Can you please assist in one more place where I have been struck for more than 1 week?

My application is written in Polymer JS and I am trying to access inner shadow elements using javascriptExecutor.

I wrote the code as below:

WebElement Root1 = driver.findElement(By.id(“Username”));
WebElement sRoot2 = expandRootElement(Root1);
String sRoot3 = sRoot2.findElement(By.cssSelector(“i18n-msg.style-scope.input-error”)).getText();

using this method:

public WebElement expandRootElement(WebElement element) {

WebElement ele = (WebElement) ((JavascriptExecutor) driver).executeScript("return arguments[0].shadowRoot", element);
System.out.println("Please");
return ele;

}

I am getting error : org.openqa.selenium.WebDriverException: Method has not yet been implemented (WARNING: The server did not provide any stacktrace information)
Command duration or timeout: 28 milliseconds

I have also added the jars selenium-api-2.21.0 in addition to the existing jars. Kindly assist… I have googled many websides but couldnt find any solution.

http://www.seleniumeasy.com/selenium-tutorials/accessing-shadow-dom-elements-with-webdriver

Thanks & Regards
Vaishnavi