Make a reftest for the flights app.

sky/tests/framework/flights-app-pixels.sky

Index: sky/tests/framework/flights-app-pixels.sky
diff --git a/sky/tests/framework/flights-app.sky b/sky/tests/framework/flights-app-pixels.sky
similarity index 61%
copy from sky/tests/framework/flights-app.sky
copy to sky/tests/framework/flights-app-pixels.sky
index e42ff36e275ee78bdd0f8258969f5dafdb00ba9e..ff06a09ebe7651fac03745a6ec64de08a814e052 100644
--- a/sky/tests/framework/flights-app.sky
+++ b/sky/tests/framework/flights-app-pixels.sky
@@ -1,10 +1,13 @@
 <sky>
+  <import src="../resources/run-after-display.sky" as="runAfterDisplay" />
   <import src="/sky/examples/flights-app/flights-app.sky" />
   <script>
   addEventListener("load", function() {
     var app = document.createElement("flights-app");
     document.querySelector("sky").appendChild(app);
-    internals.notifyTestComplete(internals.renderTreeAsText());
+    runAfterDisplay(function() {
+      internals.notifyTestComplete("");
+    });
   });
   </script>
 </sky>