diff --git a/build.json b/build.json
index f2cd87e..3b6fbc3 100644
--- a/build.json
+++ b/build.json
@@ -1,6 +1,6 @@
 [
     "../observe-js/src/observe.js",
-    "src/sidetable.js",
+    "../WeakMap/weakmap.js",
     "src/wrappers.js",
     "src/wrappers/events.js",
     "src/wrappers/NodeList.js",
diff --git a/conf/karma.conf.js b/conf/karma.conf.js
index ad5a580..d27221c 100644
--- a/conf/karma.conf.js
+++ b/conf/karma.conf.js
@@ -7,6 +7,7 @@ module.exports = function(karma) {
     // list of files / patterns to load in the browser
     files: [
       'observe-js/src/observe.js',
+      'WeakMap/weakmap.js',
       'ShadowDOM/tools/test/mocha-htmltest.js',
       'ShadowDOM/conf/mocha.conf.js',
       'ShadowDOM/node_modules/chai/chai.js',
diff --git a/shadowdom.js b/shadowdom.js
index 3ceef66..98c76b4 100644
--- a/shadowdom.js
+++ b/shadowdom.js
@@ -16,7 +16,7 @@
 
   [
     '../observe-js/src/observe.js',
-    'src/sidetable.js',
+    '../WeakMap/weakmap.js',
     'src/wrappers.js',
     'src/wrappers/events.js',
     'src/wrappers/NodeList.js',
diff --git a/src/ShadowRenderer.js b/src/ShadowRenderer.js
index 8de3fae..620390c 100644
--- a/src/ShadowRenderer.js
+++ b/src/ShadowRenderer.js
@@ -97,11 +97,11 @@
     parentNode.removeChild(node);
   }
 
-  var distributedChildNodesTable = new SideTable();
-  var eventParentsTable = new SideTable();
-  var insertionParentTable = new SideTable();
-  var rendererForHostTable = new SideTable();
-  var shadowDOMRendererTable = new SideTable();
+  var distributedChildNodesTable = new WeakMap();
+  var eventParentsTable = new WeakMap();
+  var insertionParentTable = new WeakMap();
+  var rendererForHostTable = new WeakMap();
+  var shadowDOMRendererTable = new WeakMap();
 
   function distributeChildToInsertionPoint(child, insertionPoint) {
     getDistributedChildNodes(insertionPoint).push(child);
@@ -283,7 +283,7 @@
       var newChildren = this.childNodes;
       // plain array of real nodes.
       var oldChildren = getChildNodesSnapshot(unwrap(nodeWrapper));
-      var added = opt_added || new SideTable();
+      var added = opt_added || new WeakMap();
 
       var splices = spliceDiff.calculateSplices(newChildren, oldChildren);
 
diff --git a/src/sidetable.js b/src/sidetable.js
deleted file mode 100644
index 8e783b5..0000000
--- a/src/sidetable.js
+++ /dev/null
@@ -1,40 +0,0 @@
-/*
- * Copyright 2012 The Polymer Authors. All rights reserved.
- * Use of this source code is goverened by a BSD-style
- * license that can be found in the LICENSE file.
- */
-
-// SideTable is a weak map where possible. If WeakMap is not available the
-// association is stored as an expando property.
-var SideTable;
-// TODO(arv): WeakMap does not allow for Node etc to be keys in Firefox
-if (typeof WeakMap !== 'undefined' && navigator.userAgent.indexOf('Firefox/') < 0) {
-  SideTable = WeakMap;
-} else {
-  (function() {
-    var defineProperty = Object.defineProperty;
-    var counter = Date.now() % 1e9;
-
-    SideTable = function() {
-      this.name = '__st' + (Math.random() * 1e9 >>> 0) + (counter++ + '__');
-    };
-
-    SideTable.prototype = {
-      set: function(key, value) {
-        var entry = key[this.name];
-        if (entry && entry[0] === key)
-          entry[1] = value;
-        else
-          defineProperty(key, this.name, {value: [key, value], writable: true});
-      },
-      get: function(key) {
-        var entry;
-        return (entry = key[this.name]) && entry[0] === key ?
-            entry[1] : undefined;
-      },
-      delete: function(key) {
-        this.set(key, undefined);
-      }
-    }
-  })();
-}
diff --git a/src/wrappers.js b/src/wrappers.js
index 19dc6f3..c3795c6 100644
--- a/src/wrappers.js
+++ b/src/wrappers.js
@@ -7,8 +7,8 @@ var ShadowDOMPolyfill = {};
 (function(scope) {
   'use strict';
 
-  var constructorTable = new SideTable();
-  var nativePrototypeTable = new SideTable();
+  var constructorTable = new WeakMap();
+  var nativePrototypeTable = new WeakMap();
   var wrappers = Object.create(null);
 
   // Don't test for eval if document has CSP securityPolicy object and we can
diff --git a/src/wrappers/Document.js b/src/wrappers/Document.js
index 2e09c76..1390058 100644
--- a/src/wrappers/Document.js
+++ b/src/wrappers/Document.js
@@ -21,7 +21,7 @@
   var wrapEventTargetMethods = scope.wrapEventTargetMethods;
   var wrapNodeList = scope.wrapNodeList;
 
-  var implementationTable = new SideTable();
+  var implementationTable = new WeakMap();
 
   function Document(node) {
     Node.call(this, node);
diff --git a/src/wrappers/Element.js b/src/wrappers/Element.js
index 039ff20..02797f5 100644
--- a/src/wrappers/Element.js
+++ b/src/wrappers/Element.js
@@ -16,7 +16,7 @@
   var registerWrapper = scope.registerWrapper;
   var wrappers = scope.wrappers;
 
-  var shadowRootTable = new SideTable();
+  var shadowRootTable = new WeakMap();
   var OriginalElement = window.Element;
 
 
diff --git a/src/wrappers/HTMLTemplateElement.js b/src/wrappers/HTMLTemplateElement.js
index 740abf5..fb58b47 100644
--- a/src/wrappers/HTMLTemplateElement.js
+++ b/src/wrappers/HTMLTemplateElement.js
@@ -13,8 +13,8 @@
   var unwrap = scope.unwrap;
   var wrap = scope.wrap;
 
-  var contentTable = new SideTable();
-  var templateContentsOwnerTable = new SideTable();
+  var contentTable = new WeakMap();
+  var templateContentsOwnerTable = new WeakMap();
 
   // http://dvcs.w3.org/hg/webcomponents/raw-file/tip/spec/templates/index.html#dfn-template-contents-owner
   function getTemplateContentsOwner(doc) {
diff --git a/src/wrappers/ShadowRoot.js b/src/wrappers/ShadowRoot.js
index b2e9556..75b7c78 100644
--- a/src/wrappers/ShadowRoot.js
+++ b/src/wrappers/ShadowRoot.js
@@ -13,8 +13,8 @@
   var setInnerHTML = scope.setInnerHTML;
   var unwrap = scope.unwrap;
 
-  var shadowHostTable = new SideTable();
-  var nextOlderShadowTreeTable = new SideTable();
+  var shadowHostTable = new WeakMap();
+  var nextOlderShadowTreeTable = new WeakMap();
 
   function ShadowRoot(hostWrapper) {
     var node = unwrap(hostWrapper.impl.ownerDocument.createDocumentFragment());
diff --git a/src/wrappers/events.js b/src/wrappers/events.js
index f5edc8a..c24bd33 100644
--- a/src/wrappers/events.js
+++ b/src/wrappers/events.js
@@ -12,17 +12,17 @@
   var wrap = scope.wrap;
   var wrappers = scope.wrappers;
 
-  var wrappedFuns = new SideTable();
-  var listenersTable = new SideTable();
-  var handledEventsTable = new SideTable();
-  var targetTable = new SideTable();
-  var currentTargetTable = new SideTable();
-  var relatedTargetTable = new SideTable();
-  var eventPhaseTable = new SideTable();
-  var stopPropagationTable = new SideTable();
-  var stopImmediatePropagationTable = new SideTable();
-  var eventHandlersTable = new SideTable();
-  var eventPathTable = new SideTable();
+  var wrappedFuns = new WeakMap();
+  var listenersTable = new WeakMap();
+  var handledEventsTable = new WeakMap();
+  var targetTable = new WeakMap();
+  var currentTargetTable = new WeakMap();
+  var relatedTargetTable = new WeakMap();
+  var eventPhaseTable = new WeakMap();
+  var stopPropagationTable = new WeakMap();
+  var stopImmediatePropagationTable = new WeakMap();
+  var eventHandlersTable = new WeakMap();
+  var eventPathTable = new WeakMap();
 
   function isShadowRoot(node) {
     return node instanceof wrappers.ShadowRoot;