title: Alert Dialog with Rounded corners in flutter
body: <p>I am trying to create an alert dialog with rounded corners in Flutter same as below screenshot. also add my code here, but my output is exactly different from the expected one. anyone, please help me.</p>
  
  <h3>Expected Alert Dialog</h3>
  
  <p><img src="https://i.stack.imgur.com/rNJVW.png" alt="Expected Alert Dialog"></p>
  
  <p>my code is here.</p>
  
  <pre class="lang-dart prettyprint-override"><code>void _showAlert() {
  AlertDialog dialog = new AlertDialog(
    content: new Container(
      width: 260.0,
      height: 230.0,
      decoration: new BoxDecoration(
        shape: BoxShape.rectangle,
        color: const Color(0xFFFFFF),
        borderRadius: new BorderRadius.all(new Radius.circular(32.0)),
      ),
      child: new Column(
        crossAxisAlignment: CrossAxisAlignment.stretch,
        children: &lt;Widget&gt;[
          // dialog top
          new Expanded(
            child: new Row(
              children: &lt;Widget&gt;[
                new Container(
                  // padding: new EdgeInsets.all(10.0),
                  decoration: new BoxDecoration(
                    color: Colors.white,
                  ),
                  child: new Text(
                    'Rate',
                    style: TextStyle(
                      color: Colors.black,
                      fontSize: 18.0,
                      fontFamily: 'helvetica_neue_light',
                    ),
                    textAlign: TextAlign.center,
                  ),
                ),
              ],
            ),
          ),
  
          // dialog centre
          new Expanded(
            child: new Container(
                child: new TextField(
              decoration: new InputDecoration(
                border: InputBorder.none,
                filled: false,
                contentPadding: new EdgeInsets.only(
                    left: 10.0, top: 10.0, bottom: 10.0, right: 10.0),
                hintText: ' add review',
                hintStyle: new TextStyle(
                  color: Colors.grey.shade500,
                  fontSize: 12.0,
                  fontFamily: 'helvetica_neue_light',
                ),
              ),
            )),
            flex: 2,
          ),
  
          // dialog bottom
          new Expanded(
            child: new Container(
              padding: new EdgeInsets.all(16.0),
              decoration: new BoxDecoration(
                color: const Color(0xFF33b17c),
              ),
              child: new Text(
                'Rate product',
                style: TextStyle(
                  color: Colors.white,
                  fontSize: 18.0,
                  fontFamily: 'helvetica_neue_light',
                ),
                textAlign: TextAlign.center,
              ),
            ),
          ),
        ],
      ),
    ),
  );
  
  showDialog(context: context, child: dialog);
  }
  }
  </code></pre>
  
  <h3>The output I get from the above code is.</h3>
  
  <p><img src="https://i.stack.imgur.com/Rmz7j.png" alt="screenshot"></p>