Update.

test/io/window_size.wren

@@ -0,0 +1,13 @@
+// retoor <retoor@molodetz.nl>
+
+import "io" for Stdin
+
+var size = Stdin.windowSize
+
+System.print(size == null || size is List) // expect: true
+
+if (size != null) {
+    System.print(size.count == 2)
+    System.print(size[0] > 0)
+    System.print(size[1] > 0)
+}