title: Remove marker in google_maps_flutter
body: <p>I have added <a href="https://github.com/flutter/plugins/tree/master/packages/google_maps_flutter" rel="nofollow noreferrer">Google Maps for Flutter</a> 
  i know how to add a marker as it is given clearly in there examples</p>
  
  <pre><code>MarkerOptions _options = new MarkerOptions(
            position: LatLng(
              driver_lat,
              driver_lng,
            ),
            infoWindowText:
                const InfoWindowText('An interesting location', '*'));
  
        Marker marker = new Marker('1', _options);
  
        //Adding Marker
        googleMapController.addMarker(_options);
  </code></pre>
  
  <p>And i am removing the marker like below</p>
  
  <pre><code>googleMapController.removeMarker(marker);
  </code></pre>
  
  <p>for adding the marker it is taking MarkerOptions object as a parameter but for removing the marker it is asking for Marker object as parameter and my removing marker code is not working.
  i am getting the below error</p>
  
  <pre><code>Failed assertion: line 201 pos 12: '_markers[marker._id] == marker': is not true.
  </code></pre>