Re-try of Tightening up dart2js load timing.

tools/testing/dart/browser_test.dart

Index: tools/testing/dart/browser_test.dart
diff --git a/tools/testing/dart/browser_test.dart b/tools/testing/dart/browser_test.dart
index 3d9dbd910cb702469220cc439f53b6d72c940a46..646115bf7934710a4e9d30d73625b3e81c10b1ec 100644
--- a/tools/testing/dart/browser_test.dart
+++ b/tools/testing/dart/browser_test.dart
@@ -25,7 +25,8 @@ String getHtmlContents(String title,
   <script type="text/javascript"
           src="/root_dart/pkg/unittest/lib/test_controller.js">
   </script>
-  <script type="$scriptType" src="$sourceScript" onerror="externalError(null)">
+  <script type="$scriptType" src="$sourceScript" onerror="externalError(null)"
+          defer>
   </script>
   <script type="text/javascript"
           src="/root_dart/pkg/browser/lib/dart.js"></script>
@@ -46,7 +47,7 @@ String getHtmlLayoutContents(String scriptType, Path sourceScript) =>
   <script type="text/javascript">
     if (navigator.webkitStartDart) navigator.webkitStartDart();
   </script>
-  <script type="$scriptType" src="$sourceScript"></script>
+  <script type="$scriptType" src="$sourceScript" defer></script>
 </body>
 </html>
 """;