use atFlush in UI test helpers

packages/test-helpers/onscreendiv.js

@@ -50,11 +50,9 @@ OnscreenDiv.prototype.kill = function() {
   if (self.div.parentNode)
     self.div.parentNode.removeChild(self.div);
 
-  var cx = new Meteor.deps.Context;
-  cx.on_invalidate(function() {
+  Meteor.atFlush(function () {
     Spark.finalize(self.div);
   });
-  cx.invalidate();
 };
 
 // remove the DIV from the document

packages/test-helpers/wrappedfrag.js

@@ -31,13 +31,11 @@ WrappedFrag.prototype.release = function() {
   // decrement frag's GC protection reference count
   // Clean up on flush, if hits 0.  Wait to decrement
   // so no one else cleans it up first.
-  var cx = new Meteor.deps.Context;
-  cx.on_invalidate(function() {
+  Meteor.atFlush(function () {
     if (! --frag["_protect"]) {
       Spark.finalize(frag);
     }
   });
-  cx.invalidate();
   return this;
 };