fix($window): allow $window to be mocked in unit tests

Fixes #15685

Author: Matt Lewis

src/ng/jsonpCallbacks.js

@@ -11,7 +11,7 @@
  */
 var $jsonpCallbacksProvider = /** @this */ function() {
   this.$get = ['$window', function($window) {
-    var callbacks = $window.angular.callbacks;
+    var callbacks = $window.angular ? $window.angular.callbacks : {};
     var callbackMap = {};
 
     function createCallback(callbackId) {

test/ng/jsonpCallbacksSpec.js

@@ -79,4 +79,17 @@ describe('$jsonpCallbacks', function() {
       expect($window.angular.callbacks._0).toBeUndefined();
     }));
   });
+
+  describe('$window mocked', function() {
+
+    beforeEach(module(function($provide) {
+      $provide.value('$window', {});
+    }));
+
+    it('should not throw when $window.angular does not exist', inject(function($injector) {
+      expect(function() {
+        $injector.get('$jsonpCallbacks');
+      }).not.toThrow();
+    }));
+  });
 });