title: Can't add markers to mapbox map with mapbox-gl.js
body: <p>I am desperately trying to add some markers to a mapbox map by using the mapbox-gl-js with typescript and I follwed several tutorials from the mapbox site, but it doesn't work for me.</p>
  
  <pre><code>    mapboxgl.accessToken = 'mykey';
  
      this.map = new mapboxgl.Map({
        container: 'map',
        style: this.style,
        zoom: 13,
        center: [12, 12]
      });
  
      this.map.on('load', (event) =&gt; {
  
        this.map.addSource('testSrc', {
          type: 'geojson',
          data: {
            type: 'FeatureCollection',
            features: [{
              type: 'Feature',
              geometry: {
                type: 'Point',
                coordinates: [ 12, 12]
              },
              properties: {
                title: 'Mapbox',
                description: 'Washington, D.C.'
              }
            },
              {
                type: 'Feature',
                geometry: {
                  type: 'Point',
                  coordinates: [-122.414, 37.776]
                },
                properties: {
                  title: 'Mapbox',
                  description: 'San Francisco, California'
                }
              }]
          }
    });
  
        this.map.addLayer({
          id: 'testSrc',
          source: 'testSrc',
          type: 'circle',
          layout: {
            'text-field': '{message}',
            'text-size': 24,
            'text-transform': 'uppercase',
            'icon-image': 'rocket-15',
            'text-offset': [0, 1.5]
          },
          paint: {
            'text-color': '#f16624',
            'text-halo-color': '#fff',
            'text-halo-width': 2
          }
        });
  
        this.map.addControl(new mapboxgl.NavigationControl());
  
      });
  </code></pre>
  
  <p>I created a source with some static data and I set it to a layer of the mapboxgl map-object. 
  The map is displaying without any problems, but I can't add some markers with the required geojson format. </p>
  
  <p>My goal is to dynamically add markers, but here I stuck in adding some static ones.</p>
  
  <p>Do you have any idea what's the problem in here?</p>
  
  <p>Regards Marko</p>