Add shared tests demonstrating bugs #25624, #25625, and #25626.

tests/language/accessor_conflict_export_helper.dart

Index: tests/language/accessor_conflict_export_helper.dart
diff --git a/tests/language/export_not_shadowed_by_prefix_helper.dart b/tests/language/accessor_conflict_export_helper.dart
similarity index 73%
copy from tests/language/export_not_shadowed_by_prefix_helper.dart
copy to tests/language/accessor_conflict_export_helper.dart
index 3dcd016c6b4b3b7fb5c1f29548aded677d463c3f..ff065eca870e61adec011c9db959079b7346d38a 100644
--- a/tests/language/export_not_shadowed_by_prefix_helper.dart
+++ b/tests/language/accessor_conflict_export_helper.dart
@@ -2,5 +2,5 @@
 // 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.
 
-export "export_not_shadowed_by_prefix_helper2.dart";
-import "dart:core" as f;
+export "accessor_conflict_getter.dart";
+export "accessor_conflict_setter.dart";