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chrome/test/data/webui/polymer_browser_test_base.js

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 /**
  * @fileoverview Framework for running JavaScript tests of Polymer elements.
  */
 
 /**
  * Test fixture for Polymer element testing.
@@ skipping 84 matching lines @@
     return basePath + library;
   });
 };
 
 /**
  * Allows chaining asynchronous function to avoid having to write nested
  * Promises or setTimeouts.
  * Example: PolymerTest.async(fn1).async(fn2).async(fn3).async(done);
  */
 PolymerTest.async = function(fn) {
-  // Wrap the asynchronous call to |fn| in a Promise.
-  var p = new Promise(function(resolve, reject) {
-    setTimeout(function() {
-      fn.call();
-      resolve();
+  if (!PolymerTest.lastPromise_) {
+   PolymerTest.lastPromise_ = new Promise(function(resolve) {
+     setTimeout(fn);
+     setTimeout(resolve);
+   });
+  } else {
+    PolymerTest.lastPromise_ = PolymerTest.lastPromise_.then(function() {
+      return new Promise(function(resolve) {
+        fn();
+        setTimeout(resolve);
+      });
     });
-  });
+  }
   // Return a helper function of the same name which adds this function to the
   // Promise chain.
   return {
-    async: function(fn) {
-      p.then(function() {
-        return PolymerTest.async(fn);
-      });
-    },
+    async: PolymerTest.async
   };
 };