title: Run some Javascript in Flutter Webview
body: <p>How can I run Javascript in flutter_webview_plugin? I tried with this:</p>
  <pre><code>onPressed: () {
     flutterWebviewPlugin.evalJavascript('&lt;script language=&quot;JavaScript&quot; type=&quot;text/javascript&quot;&gt;alert(&quot;Hello World&quot;)&lt;/script&gt;');
  },
  </code></pre>
  <p>But nothing happens.</p>
  <p>I thought with <code>flutterWebviewPlugin.evalJavascript</code> it's possible to run Javascript in a Webview.
  Did I do something wrong?</p>