Allow source to be set after layer construction

examples/lazy-source.js

@@ -0,0 +1,26 @@
+goog.require('ol.Map');
+goog.require('ol.View');
+goog.require('ol.layer.Tile');
+goog.require('ol.source.MapQuest');
+
+var source = new ol.source.MapQuest({layer: 'sat'});
+
+var layer = new ol.layer.Tile();
+
+var map = new ol.Map({
+  layers: [layer],
+  renderer: exampleNS.getRendererFromQueryString(),
+  target: 'map',
+  view: new ol.View({
+    center: [0, 0],
+    zoom: 2
+  })
+});
+
+document.getElementById('set-source').onclick = function() {
+  layer.setSource(source);
+};
+
+document.getElementById('unset-source').onclick = function() {
+  layer.setSource(null);
+};