diff --git a/externs/closure-types.js b/externs/closure-types.js
index c420bc296b..118f0085c6 100644
--- a/externs/closure-types.js
+++ b/externs/closure-types.js
@@ -396,9 +396,9 @@ Polymer_ElementMixin.prototype._readyClients = function(){};
 Polymer_ElementMixin.prototype._attachDom = function(dom){};
 /**
 * @override
-* @param {*} name
-* @param {*} old
-* @param {*} value
+* @param {string} name
+* @param {string} old
+* @param {string} value
 */
 Polymer_ElementMixin.prototype.attributeChangedCallback = function(name, old, value){};
 /**
@@ -437,14 +437,32 @@ function Polymer_LegacyElementMixin(){}
 */
 Polymer_LegacyElementMixin.prototype.created = function(){};
 /**
+* @override
+*/
+Polymer_LegacyElementMixin.prototype.connectedCallback = function(){};
+/**
 */
 Polymer_LegacyElementMixin.prototype.attached = function(){};
 /**
+* @override
+*/
+Polymer_LegacyElementMixin.prototype.disconnectedCallback = function(){};
+/**
 */
 Polymer_LegacyElementMixin.prototype.detached = function(){};
 /**
+* @override
+* @param {string} name
+* @param {string} old
+* @param {string} value
+*/
+Polymer_LegacyElementMixin.prototype.attributeChangedCallback = function(name, old, value){};
+/**
+* @param {string} name
+* @param {string} old
+* @param {string} value
 */
-Polymer_LegacyElementMixin.prototype.attributeChanged = function(){};
+Polymer_LegacyElementMixin.prototype.attributeChanged = function(name, old, value){};
 /**
 * @override
 */
diff --git a/lib/legacy/legacy-element-mixin.html b/lib/legacy/legacy-element-mixin.html
index e831d6c1ef..4e0b5cfec3 100644
--- a/lib/legacy/legacy-element-mixin.html
+++ b/lib/legacy/legacy-element-mixin.html
@@ -43,8 +43,8 @@
     /**
      * @constructor
      * @extends {base}
-     * @implements {Polymer_ElementMixin}
      * @implements {Polymer_GestureEventListeners}
+     * @implements {Polymer_ElementMixin}
      */
     const legacyElementBase = Polymer.GestureEventListeners(Polymer.ElementMixin(base));
 
@@ -61,7 +61,7 @@
 
     /**
      * @polymerMixinClass
-     * @implements {Polymer_LegacyElement}
+     * @implements {Polymer_LegacyElementMixin}
      */
     class LegacyElement extends legacyElementBase {
 
@@ -69,6 +69,9 @@
         super();
         this.root = this;
         this.created();
+        this.isAttached = false;
+        this.__boundListeners = null;
+        this._debouncers = null;
       }
 
       /**
@@ -77,6 +80,11 @@
        */
       created() {}
 
+      /**
+       * Provides an implementation of `connectedCallback`
+       * which adds Polymer legacy API's `attached` method.
+       * @override
+       */
       connectedCallback() {
         super.connectedCallback();
         this.isAttached = true;
@@ -89,6 +97,11 @@
        */
       attached() {}
 
+      /**
+       * Provides an implementation of `disconnectedCallback`
+       * which adds Polymer legacy API's `detached` method.
+       * @override
+       */
       disconnectedCallback() {
         super.disconnectedCallback();
         this.isAttached = false;
@@ -101,6 +114,14 @@
        */
       detached() {}
 
+      /**
+       * Provides an override implementation of `attributeChangedCallback`
+       * which adds the Polymer legacy API's `attributeChanged` method.
+       * @param {string} name Name of attribute.
+       * @param {string} old Old value of attribute.
+       * @param {string} value Current value of attribute.
+       * @override
+       */
       attributeChangedCallback(name, old, value) {
         if (old !== value) {
           super.attributeChangedCallback(name, old, value);
@@ -111,8 +132,11 @@
       /**
        * Legacy callback called during `attributeChangedChallback`, for overriding
        * by the user.
+       * @param {string} name Name of attribute.
+       * @param {string} old Old value of attribute.
+       * @param {string} value Current value of attribute.
        */
-      attributeChanged() {}
+      attributeChanged(name, old, value) {} // eslint-disable-line no-unused-vars
 
       /**
        * Overrides the default `Polymer.PropertyEffects` implementation to
@@ -188,7 +212,7 @@
        * @return {string} Serialized value
        */
       serialize(value) {
-        return this._serializeValue(value);
+        return this._serializeValue(value) || '';
       }
 
       /**
@@ -421,7 +445,7 @@
        */
       get domHost() {
         let root = this.getRootNode();
-        return (root instanceof DocumentFragment) ? root.host : root;
+        return (root instanceof DocumentFragment) ? /** @type {ShadowRoot} */ (root).host : root;
       }
 
       /**
@@ -433,8 +457,8 @@
        * that contains an insertion point (<slot>) inside its subtree.
        */
       distributeContent() {
-        if (window.ShadyDOM && this.shadowRoot) {
-          this.shadowRoot.forceRender();
+        if (window.ShadyDOM) {
+          window.ShadyDOM.flush();
         }
       }
 
diff --git a/lib/legacy/polymer.dom.html b/lib/legacy/polymer.dom.html
index ae9ba4bbc1..0d7f62dc8d 100644
--- a/lib/legacy/polymer.dom.html
+++ b/lib/legacy/polymer.dom.html
@@ -15,6 +15,9 @@
   'use strict';
 
   const p = Element.prototype;
+  /**
+   * @const {function(this:Element, string): boolean}
+   */
   const normalizedMatchesSelector = p.matches || p.matchesSelector ||
     p.mozMatchesSelector || p.msMatchesSelector ||
     p.oMatchesSelector || p.webkitMatchesSelector;
@@ -24,7 +27,7 @@
    *
    * @function matchesSelector
    * @memberof Polymer.dom
-   * @param {Node} node Node to check selector against
+   * @param {!Element} node Node to check selector against
    * @param {string} selector Selector to match
    * @return {boolean} True if node matched selector
    */
@@ -38,6 +41,9 @@
    */
   class DomApi {
 
+    /**
+     * @param {Node} node Node for which to create a Polymer.dom helper object.
+     */
     constructor(node) {
       this.node = node;
     }
@@ -146,10 +152,8 @@
     }
 
     /**
-     * Returns a flattened list of all child nodes and nodes distributed
+     * @return {Array} Returns a flattened list of all child nodes and nodes assigned
      * to child slots.
-     *
-     * @return {type} Description
      */
     getEffectiveChildNodes() {
       return Polymer.FlattenedNodesObserver.getFlattenedNodes(this.node);
@@ -189,6 +193,10 @@
   function forwardMethods(proto, methods) {
     for (let i=0; i < methods.length; i++) {
       let method = methods[i];
+      /**
+       * @this {DomApi}
+       * @return {*} Returns method value
+       */
       proto[method] = function() {
         return this.node[method].apply(this.node, arguments);
       }
@@ -199,6 +207,10 @@
     for (let i=0; i < properties.length; i++) {
       let name = properties[i];
       Object.defineProperty(proto, name, {
+        /**
+         * @this {DomApi}
+         * @return {*} Returns property value
+         */
         get: function() {
           return this.node[name];
         },
@@ -288,14 +300,19 @@
    * @summary Legacy DOM and Event manipulation API wrapper factory used to
    * abstract differences between native Shadow DOM and "Shady DOM."
    * @memberof Polymer
-   * @param {Node|Event} obj Node or event to operate on
+   * @param {!Node|Event} obj Node or event to operate on
    * @return {DomApi|EventApi} Wrapper providing either node API or event API
    */
   Polymer.dom = function(obj) {
     obj = obj || document;
-    let ctor = obj instanceof Event ? EventApi : DomApi;
     if (!obj.__domApi) {
-      obj.__domApi = new ctor(obj);
+      let helper;
+      if (obj instanceof Event) {
+        helper = new EventApi(obj);
+      } else {
+        helper = new DomApi(obj);
+      }
+      obj.__domApi = helper;
     }
     return obj.__domApi;
   };
diff --git a/lib/mixins/element-mixin.html b/lib/mixins/element-mixin.html
index 92b728d714..4bef530fb2 100644
--- a/lib/mixins/element-mixin.html
+++ b/lib/mixins/element-mixin.html
@@ -717,6 +717,9 @@
        * same name.  "Dash-cased" attributes are deserialzed to "camelCase"
        * properties.
        *
+       * @param {string} name Name of attribute.
+       * @param {string} old Old value of attribute.
+       * @param {string} value Current value of attribute.
        * @override
        */
       attributeChangedCallback(name, old, value) {