step 1 in making unittest platform independent.

samples/tests/samples/src/swarm/SwarmTest.dart

Index: samples/tests/samples/src/swarm/SwarmTest.dart
diff --git a/samples/tests/samples/src/swarm/SwarmTest.dart b/samples/tests/samples/src/swarm/SwarmTest.dart
index 437f32996fa2c3fe632da0fa164313c84037947c..628ae034f86e5fd43aa95b3f363132bfbefa59ef 100644
--- a/samples/tests/samples/src/swarm/SwarmTest.dart
+++ b/samples/tests/samples/src/swarm/SwarmTest.dart
@@ -45,7 +45,7 @@ void main() {
   }
 
   asyncTest('BackButton', 1, () {
-    serialInvokeAsync([() {
+    _serialInvokeAsync([() {
       Expect.equals(null, swarm.frontView.storyView); // verify initial state
 
       // Make sure we've transitioned to the section
@@ -137,3 +137,18 @@ class UIStateProxy extends SwarmState {
     clearHistory();
   }
 }
+
+void _serialInvokeAsync(List closures) {
+  final length = closures.length;
+  if (length > 0) {
+    int i = 0;
+    void invokeNext() {
+      closures[i]();
+      i++;
+      if (i < length) {
+        window.setTimeout(invokeNext, 0);
+      }
+    }
+    window.setTimeout(invokeNext, 0);
+  }
+}