Added tests for interfaces that extend other interfaces.

Exercises IsAssignableFrom and FindInterfaceMethod. Also made
Class::Implements private again and switched to using IsAssignableFrom.

Change-Id: Iea195a2cc124a87ebb3d87d778a7edcd25984b46
diff --git a/src/object.h b/src/object.h
index 9106472..f26d9ef 100644
--- a/src/object.h
+++ b/src/object.h
@@ -1939,9 +1939,8 @@
                              new_source_file, false);
   }
 
-  bool Implements(const Class* klass) const;
-
  private:
+  bool Implements(const Class* klass) const;
   bool IsArrayAssignableFromArray(const Class* klass) const;
   bool IsAssignableFromArray(const Class* klass) const;
   bool IsSubClass(const Class* klass) const;