Add mirrors tests

test/codegen/lib/mirrors/library_imports_prefixed.dart

Index: test/codegen/lib/mirrors/library_imports_prefixed.dart
diff --git a/test/codegen/language/async_backwards_compatibility_2_test.dart b/test/codegen/lib/mirrors/library_imports_prefixed.dart
similarity index 59%
copy from test/codegen/language/async_backwards_compatibility_2_test.dart
copy to test/codegen/lib/mirrors/library_imports_prefixed.dart
index 4f9074f3eafc7f1433be335402d556b0a835da9b..ad50f424766e6004437ffed6b4b4dd7ac25425b9 100644
--- a/test/codegen/language/async_backwards_compatibility_2_test.dart
+++ b/test/codegen/lib/mirrors/library_imports_prefixed.dart
@@ -2,14 +2,9 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-int get async { return 1; }
+library library_imports_prefixed;
 
-class A {
-  async() => null;
-}
+import 'library_imports_a.dart' as prefixa;
+import 'library_imports_b.dart' as prefixb;
 
-main() {
-  var a = async;
-  var b = new A();
-  var c = b.async();
-}
+var somethingFromPrefixed;