diff -r 000000000000 -r 6474c204b198 js/src/jit-test/tests/collections/Set-size.js
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/js/src/jit-test/tests/collections/Set-size.js	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,7 @@
+// Each Set has its own size.
+
+var s1 = Set(), s2 = Set();
+for (var i = 0; i < 30; i++)
+    s1.add(i);
+assertEq(s1.size, 30);
+assertEq(s2.size, 0);