add @MirrorsUsed to observe/polymer/polymer_exprs pkgs

pkg/polymer/test/instance_attrs_test.dart

Index: pkg/polymer/test/instance_attrs_test.dart
diff --git a/pkg/polymer/test/instance_attrs_test.dart b/pkg/polymer/test/instance_attrs_test.dart
index ebe17b8e3a89f52afb62492b4c5ae6cfca110d35..71030797992954669cf2591aa5375da681031c35 100644
--- a/pkg/polymer/test/instance_attrs_test.dart
+++ b/pkg/polymer/test/instance_attrs_test.dart
@@ -7,6 +7,12 @@ import 'package:unittest/unittest.dart';
 import 'package:unittest/html_config.dart';
 import 'package:polymer/polymer.dart';
 
+// Note: we use @CustomTag to make this type reflectable.
+@CustomTag('my-element')
+class MyElement extends PolymerElement {
+  MyElement.created() : super.created();
+}
+
 main() {
   useHtmlConfiguration();