add @MirrorsUsed to observe/polymer/polymer_exprs pkgs

pkg/polymer/test/build/script_compactor_test.dart

Index: pkg/polymer/test/build/script_compactor_test.dart
diff --git a/pkg/polymer/test/build/script_compactor_test.dart b/pkg/polymer/test/build/script_compactor_test.dart
index 7f0c39030fd3cc0c3feec231ce4a1de295e20710..e0977bebeb9a08d53c520481d6b2f645d758504b 100644
--- a/pkg/polymer/test/build/script_compactor_test.dart
+++ b/pkg/polymer/test/build/script_compactor_test.dart
@@ -43,11 +43,7 @@ void main() {
           '</body></html>',
 
       'a|web/test.html_bootstrap.dart':
-          '''library app_bootstrap;
-
-          import 'package:polymer/polymer.dart';
-          import 'dart:mirrors' show currentMirrorSystem;
-
+          '''$MAIN_HEADER
           import 'a.dart' as i0;
 
           void main() {
@@ -76,11 +72,7 @@ void main() {
           '</body></html>',
 
       'a|web/test.html_bootstrap.dart':
-          '''library app_bootstrap;
-
-          import 'package:polymer/polymer.dart';
-          import 'dart:mirrors' show currentMirrorSystem;
-
+          '''$MAIN_HEADER
           import 'a.dart' as i0;
           import 'b.dart' as i1;
           import 'c.dart' as i2;