Add test for "TreeShaking: Detect instance fields never written"

Bug: 80455722
Change-Id: I65de4c34eafb982fa85f7081613ed7d71b3e959a
diff --git a/src/test/examples/shaking18/Base.java b/src/test/examples/shaking18/Base.java
new file mode 100644
index 0000000..f5707d1
--- /dev/null
+++ b/src/test/examples/shaking18/Base.java
@@ -0,0 +1,9 @@
+// Copyright (c) 2018, the R8 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.
+package shaking18;
+
+public abstract class Base {
+
+  public abstract String getMessage();
+}