Teach the network simulator to wait for Promises.

Tools/GardeningServer/model/test/tree-status-tests.html

 <!--
 Copyright 2014 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.
 -->
 
 <link rel="import" href="../tree-status.html">
 
 <link rel="import" href="../../lib/network-simulator.html">
 
@@ skipping 35 matching lines @@
     simulator.json = function(url) {
       if (url.indexOf('closed') != -1)
         return Promise.resolve(closedTreeJson);
       else if (url.indexOf('throttled') != -1)
         return Promise.resolve(throttledTreeJson);
       else
         return Promise.resolve(openTreeJson);
     };
 
     simulator.runTest(function() {
-      Promise.all([
+      return Promise.all([
         opentree.update().then(function() {
           assert.equal(opentree.status, 'open');
         }),
         throttledtree.update().then(function() {
           assert.equal(throttledtree.message,
                        "Tree is throttled just for fun by username@test.org");
           assert.equal(throttledtree.status, "throttled");
         }),
         closedtree.update().then(function() {
           assert.equal(closedtree.message,
                        "Tree is closed by ojan@chromium.org");
           assert.equal(closedtree.status, "closed");
         })
       ]);
-    }).then(done);
+    });
   });
 });
 
 })();
 </script>