Provide a polyfill for window.requestAnimationFrame() in tests

rAF is not available in PhantomJS 1.x but is supported by all of our
target browsers (IE >= 10).

h/static/scripts/directive/test/excerpt-test.js

@@ -41,21 +41,9 @@ describe('excerpt directive', function () {
     return el.querySelector('.excerpt').offsetHeight;
   }
 
-  var defaultRAF = window.requestAnimationFrame;
-
   before(function () {
     angular.module('app', [])
       .directive('excerpt', excerpt.directive);
-
-    // requestAnimationFrame() is used internally by <excerpt>
-    // to schedule overflow state checks
-    window.requestAnimationFrame = function (callback) {
-      setTimeout(callback, 0);
-    };
-  });
-
-  after(function () {
-    window.requestAnimationFrame = defaultRAF;
   });
 
   beforeEach(function () {

h/static/scripts/karma-phantomjs-polyfill.js

@@ -11,6 +11,9 @@
 // by all browsers we support (IE >= 10)
 require('core-js/es5');
 
+// PhantomJS 1.x does not support rAF.
+require('raf').polyfill();
+
 // Additional polyfills for newer features.
 // Be careful that any polyfills used here match what is used in the
 // app itself.