Update the README's "pub serve" section for polymer compatibility.

README.md

 `test` provides a standard way of writing and running tests in Dart.
 
 ## Writing Tests
 
 Tests are specified using the top-level [`test()`][test] function, and test
 assertions are made using [`expect()`][expect]:
 
 [test]: http://www.dartdocs.org/documentation/test/latest/index.html#test/test@id_test
 [expect]: http://www.dartdocs.org/documentation/test/latest/index.html#test/test@id_expect
 
@@ skipping 437 matching lines @@
 instance rather than from the filesystem. **This feature is only supported on
 Dart `1.9.2` and higher.**
 
 Before using the `--pub-serve` option, add the `test/pub_serve` transformer to
 your `pubspec.yaml`. This transformer adds the necessary bootstrapping code that
 allows the test runner to load your tests properly:
 
 ```yaml
 transformers:
 - test/pub_serve:
-    $include: test/**_test.dart
+    $include: test/**_test{.*,}.dart
+```
+
+Note that if you're using the test runner along with [`polymer`][polymer], you
+have to make sure that the `test/pub_serve` transformer comes *after* the
+`polymer` transformer:
+
+[polymer]: https://www.dartlang.org/polymer/
+
+```yaml
+transformer:
+- polymer
+- test/pub_serve:
+    $include: test/**_test{.*,}.dart
 ```
 
 Then, start up `pub serve`. Make sure to pay attention to which port it's using
 to serve your `test/` directory:
 
 ```shell
 $ pub serve
 Loading source assets...
 Loading test/pub_serve transformers...
 Serving my_app web on http://localhost:8080
 Serving my_app test on http://localhost:8081
 Build completed successfully
 ```
 
 In this case, the port is `8081`. In another terminal, pass this port to
 `--pub-serve` and otherwise invoke `pub run test:test` as normal:
 
 ```shell
 $ pub run test:test --pub-serve=8081 -p chrome
 "pub serve" is compiling test/my_app_test.dart...
 "pub serve" is compiling test/utils_test.dart...
 00:00 +42: All tests passed!
 ```