TileAnimation: Fix error thrown if map unset during preloading

- A layer cannot preload without a map. So when the
  masterLayer's map was unset, layer.preload threw and error.
  Fixed this by caching the masterLayer's map when preloading
  starts.
  Calling TileAnimation#preload with no map set to begin with will
  still throw an error (expected behavior).

src/maps/animations/tileanimation.js

@@ -156,7 +156,7 @@ define([
    */
   TileAnimation.prototype.preload = function() {
     var promiseToPreload = new Promise();
-
+    var mapToUseForPreloading = this.masterLayer_.getMap();
 
     // We need our times (and timeLayers)
     // to be loaded, before we can preload layers
@@ -165,7 +165,9 @@ define([
         var layers = _.values(this.timeLayers_);
 
         // Preload each layer in sequece
-        Promise.sequence(layers, this.preloadLayer_.bind(this)).
+        Promise.sequence(layers, function(layer) {
+          return layer.preload(mapToUseForPreloading);
+        }).
           done(promiseToPreload.resolve).
           fail(promiseToPreload.reject);
       }, this).

tests/spec/aeris/maps/animations/tileanimation.js

@@ -677,9 +677,11 @@ define([
       });
 
       function loadAll(timeLayers) {
-        _.each(timeLayers, function(lyr) {
-          lyr.promiseToPreload.resolve();
-        });
+        _.each(timeLayers, loadLayer);
+      }
+
+      function loadLayer(layer) {
+        layer.promiseToPreload.resolve();
       }
 
 
@@ -745,6 +747,20 @@ define([
           expect(onReject).toHaveBeenCalled();
         });
 
+        describe('if the masterLayer\'s map is unset during preloading', function() {
+
+          it('should continue to use the original map object for preloading', function() {
+            masterLayer.setMap(map);
+            animation.preload();
+
+            masterLayer.setMap(null);
+            loadLayer(timeLayers[0]);
+
+            expect(timeLayers[1].preload).toHaveBeenCalledWith(map);
+          });
+
+        });
+
       });