diff -r 000000000000 -r 6474c204b198 addon-sdk/source/lib/sdk/lang/weak-set.js
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/addon-sdk/source/lib/sdk/lang/weak-set.js	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,70 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+module.metadata = {
+  "stability": "experimental"
+};
+
+"use strict";
+
+const { Cu } = require("chrome");
+
+function makeGetterFor(Type) {
+  let cache = new WeakMap();
+
+  return function getFor(target) {
+    if (!cache.has(target))
+      cache.set(target, new Type());
+
+    return cache.get(target);
+  }
+}
+
+let getLookupFor = makeGetterFor(WeakMap);
+let getRefsFor = makeGetterFor(Set);
+
+function add(target, value) {
+  if (has(target, value))
+    return;
+
+  getLookupFor(target).set(value, true);
+  getRefsFor(target).add(Cu.getWeakReference(value));
+}
+exports.add = add;
+
+function remove(target, value) {
+  getLookupFor(target).delete(value);
+}
+exports.remove = remove;
+
+function has(target, value) {
+  return getLookupFor(target).has(value);
+}
+exports.has = has;
+
+function clear(target) {
+  getLookupFor(target).clear();
+  getRefsFor(target).clear();
+}
+exports.clear = clear;
+
+function iterator(target) {
+  let refs = getRefsFor(target);
+
+  for (let ref of refs) {
+    let value = ref.get();
+
+    // If `value` is already gc'ed, it would be `null`.
+    // The `has` function is using a WeakMap as lookup table, so passing `null`
+    // would raise an exception because WeakMap accepts as value only non-null
+    // object.
+    // Plus, if `value` is already gc'ed, we do not have to take it in account
+    // during the iteration, and remove it from the references.
+    if (value !== null && has(target, value))
+      yield value;
+    else
+      refs.delete(ref);
+  }
+}
+exports.iterator = iterator;