Superclass calls in closures.

test/super/closure.wren

@@ -0,0 +1,16 @@
+class Base {
+  toString { return "Base" }
+}
+
+class Derived is Base {
+  getClosure {
+    return fn {
+      return super.toString
+    }
+  }
+
+  toString { return "Derived" }
+}
+
+var closure = (new Derived).getClosure
+IO.write(closure.call) // expect: Base