title: Flutter onPressed of a Button on a ListView gets called automatically (when it becomes visible). Is this a bug?
body: <p>As the title mentions <code>onPressed</code> of a Button on a ListView gets called automatically</p>
  
  <p>I currently have something like this (this is a rough sketch of the code).
  Basically whenever there is a button in each row and whenever a button gets displayed on the screen its onClick is called. Not sure if this is a bug in flutter or I am doing something wrong any suggestions ?</p>
  
  <pre><code>class ModelEmployeeRow extends StatelessWidget
  {
  
      dynamic getInviteButton(String text, {var lambda,var borderRadius,var height})
      {
          final skillTextStyle = baseTextStyle.copyWith(
                  color: Colors.white,//const Color(0xffb6b2df),
                  fontSize: 11.0,
                  fontWeight: FontWeight.w200
                  );
  
          var container = new Container(
              alignment: Alignment.center,
              margin:EdgeInsets.fromLTRB(0.0,0.0,100.0,0.0),
              padding:EdgeInsets.fromLTRB(0.0,5.0,0.0,5.0),
              decoration: new BoxDecoration(
                  borderRadius: new BorderRadius.all(new Radius.circular(4.0)),
                  color: Colors.green
                  ),
              child: new Text(text, style:skillTextStyle),
              );
  
  
          var button = new FlatButton(
                  onPressed: inviteClicked(employee),
                  child: container
                  );
  
          return button;
      }
  
  
      @override
      Widget build(BuildContext context)
      {
          var stacked =  new Stack(
              children: &lt;Widget&gt;
              [
                  //mainContainer,
                  getInviteButton("Test"),
                  employeeThumbnail,
              ],
              );
  
          return new Container(
              child: stacked,
              );
  
      }
  
  }
  </code></pre>
  
  <p>and the list view side is this</p>
  
  <pre><code> var emplyeeListView = new ListView.builder(
              itemCount: employeeListShared.length,
              padding: new EdgeInsets.symmetric(vertical: 16.0),
              itemBuilder: (context, index) {
                  return new ModelEmployeeRow(employeeListShared[index]);
              },
              );
  </code></pre>
  
  <p>Now when ever a row gets visible the onclick of the button is called.</p>