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

tests/language/accessor_conflict_export_test.dart

+// Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
+// 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.
+
+// Verify that a getter and its corresponding setter can be imported from two
+// different files via a common export.  In this test the getter is imported
+// first.
+
+import "package:expect/expect.dart";
+
+import "accessor_conflict_export_helper.dart";
+
+main() {
+  getValue = 123;
+  Expect.equals(x, 123);
+  x = 456;
+  Expect.equals(setValue, 456);
+}