diff -r 000000000000 -r 6474c204b198 js/src/jit-test/tests/collections/Map-set-size.js
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/js/src/jit-test/tests/collections/Map-set-size.js	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,14 @@
+// map.set(k, v) increments the map size iff map didn't already have an entry for k.
+
+var m = Map();
+m.set('a', 0);
+assertEq(m.size, 1);
+m.set('a', 0);
+assertEq(m.size, 1);
+m.set('a', undefined);
+assertEq(m.size, 1);
+
+m.set('b', 2);
+assertEq(m.size, 2);
+m.set('a', 1);
+assertEq(m.size, 2);