Trying to prove that WeakMap is actually weak

I don't have a ton of experience with weak maps, but I would expect the following to work:

// File: example.js
var key = {};
var indirectReference = {};
indirectReference['key'] = (function() { var m = {}; m['foo'] = 'bar'; return m; })();
var map = new WeakMap();

map.set(key, indirectReference['key']);
console.log('Has key after setting value: %s', map.has(key));

delete indirectReference['key'];
console.log('Has key after deleting value: %s', map.has(key));

global.gc();
console.log('Has key after performing global.gc(): %s', map.has(key));

I downloaded the latest version of io.js (so that I could have a JavaScript runtime with WeakMap and global.gc() and ran it as follows:

./iojs --expose-gc example.js

Here is what I see:

Has key after setting value: true
Has key after deleting value: true
Has key after performing global.gc(): true

Despite my best efforts, I can't seem to get WeakMap to give up the value that is mapped to key. Am I doing it wrong? Obviously I'm making some assumptions here, so I'm curious where I'm off.

Ultimately, I would like to be able to use WeakMap to write some tests to ensure certain objects get garbage collected and don't leak memory.