Fixing html/worker_test on IE9

tests/html/worker_test.dart

Index: tests/html/worker_test.dart
diff --git a/tests/html/worker_test.dart b/tests/html/worker_test.dart
index 296391c1ac0412b60ab527a71ec3bbbb64789026..81b8558d96aa035981f8808555b03dcbcb3ec47d 100644
--- a/tests/html/worker_test.dart
+++ b/tests/html/worker_test.dart
@@ -28,6 +28,10 @@ main() {
       }, expectation);
     });
 
+    if (!Worker.supported) {
+      return;
+    }
+
     test('works', () {
       // Use Blob to make a local URL so we don't have to have a separate file.
       var blob = new Blob([workerScript], 'text/javascript');