diff --git a/.travis.yml b/.travis.yml
index 35dbe4f181..c3c05030e0 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -3,7 +3,7 @@ sudo: false
 dist: trusty
 node_js: '9'
 addons:
-  firefox: latest
+  firefox: "66.0"
   chrome: stable
 cache:
   directories:
diff --git a/lib/elements/custom-style.js b/lib/elements/custom-style.js
index f675393118..1a43b318e0 100644
--- a/lib/elements/custom-style.js
+++ b/lib/elements/custom-style.js
@@ -87,6 +87,7 @@ export class CustomStyle extends HTMLElement {
     const include = style.getAttribute(attr);
     if (include) {
       style.removeAttribute(attr);
+      /** @suppress {deprecated} */
       style.textContent = cssFromModules(include) + style.textContent;
     }
     /*
diff --git a/lib/elements/dom-bind.js b/lib/elements/dom-bind.js
index cc9b4f70ba..33e7293b09 100644
--- a/lib/elements/dom-bind.js
+++ b/lib/elements/dom-bind.js
@@ -62,11 +62,16 @@ export class DomBind extends domBindBase {
     this.__children = null;
   }
 
+  /* eslint-disable no-unused-vars */
   /**
    * @override
+   * @param {string} name Name of attribute that changed
+   * @param {?string} old Old attribute value
+   * @param {?string} value New attribute value
+   * @param {?string} namespace Attribute namespace.
    * @return {void}
    */
-  attributeChangedCallback() {
+  attributeChangedCallback(name, old, value, namespace) {
     // assumes only one observed attribute
     this.mutableData = true;
   }
diff --git a/lib/elements/dom-repeat.js b/lib/elements/dom-repeat.js
index 7b0407d2e0..2983e4ba36 100644
--- a/lib/elements/dom-repeat.js
+++ b/lib/elements/dom-repeat.js
@@ -297,7 +297,7 @@ export class DomRepeat extends domRepeatBase {
     this.__sortFn = null;
     this.__filterFn = null;
     this.__observePaths = null;
-    /** @type {?function(new:Polymer.TemplateInstanceBase, *)} */
+    /** @type {?function(new:TemplateInstanceBase, *)} */
     this.__ctor = null;
     this.__isDetached = true;
     this.template = null;
diff --git a/lib/mixins/dir-mixin.js b/lib/mixins/dir-mixin.js
index 0b2326fdc4..1957bb489c 100644
--- a/lib/mixins/dir-mixin.js
+++ b/lib/mixins/dir-mixin.js
@@ -83,6 +83,9 @@ function takeRecords() {
  * @mixinFunction
  * @polymer
  * @appliesMixin PropertyAccessors
+ * @template T
+ * @param {function(new:T)} superClass Class to apply mixin to.
+ * @return {function(new:T)} superClass with mixin applied.
  */
 export const DirMixin = dedupingMixin((base) => {
 
diff --git a/lib/mixins/disable-upgrade-mixin.js b/lib/mixins/disable-upgrade-mixin.js
index a97aa9e9ba..8b2022a6eb 100644
--- a/lib/mixins/disable-upgrade-mixin.js
+++ b/lib/mixins/disable-upgrade-mixin.js
@@ -38,6 +38,9 @@ const DISABLED_ATTR = 'disable-upgrade';
  * @mixinFunction
  * @polymer
  * @appliesMixin ElementMixin
+ * @template T
+ * @param {function(new:T)} superClass Class to apply mixin to.
+ * @return {function(new:T)} superClass with mixin applied.
  */
 export const DisableUpgradeMixin = dedupingMixin((base) => {
   /**
@@ -67,8 +70,8 @@ export const DisableUpgradeMixin = dedupingMixin((base) => {
      * @param {string} name Attribute name.
      * @param {?string} old The previous value for the attribute.
      * @param {?string} value The new value for the attribute.
-     * @param {?string=} namespace The XML namespace for the attribute.
-     * @return {undefined}
+     * @param {?string} namespace The XML namespace for the attribute.
+     * @return {void}
      */
     attributeChangedCallback(name, old, value, namespace) {
       if (name == DISABLED_ATTR) {
diff --git a/lib/mixins/element-mixin.js b/lib/mixins/element-mixin.js
index ab10624cc2..522914b33b 100644
--- a/lib/mixins/element-mixin.js
+++ b/lib/mixins/element-mixin.js
@@ -95,6 +95,9 @@ const builtCSS = window.ShadyCSS && window.ShadyCSS['cssBuild'];
  *   import strategies.
  * @summary Element class mixin that provides the core API for Polymer's
  * meta-programming features.
+ * @template T
+ * @param {function(new:T)} superClass Class to apply mixin to.
+ * @return {function(new:T)} superClass with mixin applied.
  */
 export const ElementMixin = dedupingMixin(base => {
   /**
diff --git a/lib/mixins/gesture-event-listeners.js b/lib/mixins/gesture-event-listeners.js
index b509e8db87..71a4577ff7 100644
--- a/lib/mixins/gesture-event-listeners.js
+++ b/lib/mixins/gesture-event-listeners.js
@@ -24,63 +24,48 @@ import { addListener, removeListener } from '../utils/gestures.js';
  * @mixinFunction
  * @polymer
  * @summary Element class mixin that provides API for adding Polymer's
- *   cross-platform
- * gesture events to nodes
+ *   cross-platform gesture events to nodes
+ * @template T
+ * @param {function(new:T)} superClass Class to apply mixin to.
+ * @return {function(new:T)} superClass with mixin applied.
  */
-const _GestureEventListeners = dedupingMixin(
+export const GestureEventListeners = dedupingMixin((superClass) => {
+  /**
+   * @polymer
+   * @mixinClass
+   * @implements {Polymer_GestureEventListeners}
+   */
+  class GestureEventListeners extends superClass {
     /**
-     * @template T
-     * @param {function(new:T)} superClass Class to apply mixin to.
-     * @return {function(new:T)} superClass with mixin applied.
+     * Add the event listener to the node if it is a gestures event.
+     *
+     * @param {!EventTarget} node Node to add event listener to
+     * @param {string} eventName Name of event
+     * @param {function(!Event):void} handler Listener function to add
+     * @return {void}
+     * @override
      */
-    (superClass) => {
-      /**
-       * @polymer
-       * @mixinClass
-       * @implements {Polymer_GestureEventListeners}
-       */
-      class GestureEventListeners extends superClass {
-        /**
-         * Add the event listener to the node if it is a gestures event.
-         *
-         * @param {!EventTarget} node Node to add event listener to
-         * @param {string} eventName Name of event
-         * @param {function(!Event):void} handler Listener function to add
-         * @return {void}
-         * @override
-         */
-        _addEventListenerToNode(node, eventName, handler) {
-          if (!addListener(node, eventName, handler)) {
-            super._addEventListenerToNode(node, eventName, handler);
-          }
-        }
-
-        /**
-         * Remove the event listener to the node if it is a gestures event.
-         *
-         * @param {!EventTarget} node Node to remove event listener from
-         * @param {string} eventName Name of event
-         * @param {function(!Event):void} handler Listener function to remove
-         * @return {void}
-         * @override
-         */
-        _removeEventListenerFromNode(node, eventName, handler) {
-          if (!removeListener(node, eventName, handler)) {
-            super._removeEventListenerFromNode(node, eventName, handler);
-          }
-        }
+    _addEventListenerToNode(node, eventName, handler) {
+      if (!addListener(node, eventName, handler)) {
+        super._addEventListenerToNode(node, eventName, handler);
       }
+    }
 
-      return GestureEventListeners;
-    });
+    /**
+     * Remove the event listener to the node if it is a gestures event.
+     *
+     * @param {!EventTarget} node Node to remove event listener from
+     * @param {string} eventName Name of event
+     * @param {function(!Event):void} handler Listener function to remove
+     * @return {void}
+     * @override
+     */
+    _removeEventListenerFromNode(node, eventName, handler) {
+      if (!removeListener(node, eventName, handler)) {
+        super._removeEventListenerFromNode(node, eventName, handler);
+      }
+    }
+  }
 
-// Somehow _GestureEventListeners is incorrectly typed as *. For now add this
-// cast.
-/**
- * @template T
- * @param {function(new:T)} superClass Class to apply mixin to.
- * @return {function(new:T)} superClass with mixin applied.
- */
-export const GestureEventListeners = function(superClass) {
-  return _GestureEventListeners(superClass);
-};
+  return GestureEventListeners;
+});
diff --git a/lib/mixins/mutable-data.js b/lib/mixins/mutable-data.js
index d38b5119b9..12072ac47a 100644
--- a/lib/mixins/mutable-data.js
+++ b/lib/mixins/mutable-data.js
@@ -67,6 +67,9 @@ function mutablePropertyChange(inst, property, value, old, mutableData) {
  * @polymer
  * @summary Element class mixin to skip strict dirty-checking for objects
  *   and arrays
+ * @template T
+ * @param {function(new:T)} superClass Class to apply mixin to.
+ * @return {function(new:T)} superClass with mixin applied.
  */
 export const MutableData = dedupingMixin(superClass => {
 
diff --git a/lib/mixins/properties-changed.js b/lib/mixins/properties-changed.js
index b81844b828..ff49a5746b 100644
--- a/lib/mixins/properties-changed.js
+++ b/lib/mixins/properties-changed.js
@@ -33,6 +33,9 @@ const microtask = microTask;
  * @polymer
  * @summary Element class mixin for reacting to property changes from
  *   generated property accessors.
+ * @template T
+ * @param {function(new:T)} superClass Class to apply mixin to.
+ * @return {function(new:T)} superClass with mixin applied.
  */
 export const PropertiesChanged = dedupingMixin(
     /**
@@ -161,7 +164,7 @@ export const PropertiesChanged = dedupingMixin(
 
     constructor() {
       super();
-      /** @protected {boolean} */
+      /** @type {boolean} */
       this.__dataEnabled = false;
       this.__dataReady = false;
       this.__dataInvalid = false;
@@ -426,7 +429,7 @@ export const PropertiesChanged = dedupingMixin(
      * @param {string} name Name of attribute that changed
      * @param {?string} old Old attribute value
      * @param {?string} value New attribute value
-     * @param {?string=} namespace Attribute namespace.
+     * @param {?string} namespace Attribute namespace.
      * @return {void}
      * @suppress {missingProperties} Super may or may not implement the callback
      * @override
diff --git a/lib/mixins/properties-mixin.js b/lib/mixins/properties-mixin.js
index d450cfd299..06fe449d38 100644
--- a/lib/mixins/properties-mixin.js
+++ b/lib/mixins/properties-mixin.js
@@ -47,6 +47,9 @@ function normalizeProperties(props) {
  * @appliesMixin PropertiesChanged
  * @summary Mixin that provides a minimal starting point for using
  * the PropertiesChanged mixin by providing a declarative `properties` object.
+ * @template T
+ * @param {function(new:T)} superClass Class to apply mixin to.
+ * @return {function(new:T)} superClass with mixin applied.
  */
 export const PropertiesMixin = dedupingMixin(superClass => {
 
diff --git a/lib/mixins/property-accessors.js b/lib/mixins/property-accessors.js
index 16d039ef82..2590165c03 100644
--- a/lib/mixins/property-accessors.js
+++ b/lib/mixins/property-accessors.js
@@ -91,6 +91,9 @@ function saveAccessorValue(model, property) {
  * @appliesMixin PropertiesChanged
  * @summary Element class mixin for reacting to property changes from
  *   generated property accessors.
+ * @template T
+ * @param {function(new:T)} superClass Class to apply mixin to.
+ * @return {function(new:T)} superClass with mixin applied.
  */
 export const PropertyAccessors = dedupingMixin(superClass => {
 
diff --git a/lib/mixins/property-effects.js b/lib/mixins/property-effects.js
index c12c9870d7..7fd90836d8 100644
--- a/lib/mixins/property-effects.js
+++ b/lib/mixins/property-effects.js
@@ -1079,6 +1079,9 @@ function upper(name) {
  * @appliesMixin PropertyAccessors
  * @summary Element class mixin that provides meta-programming for Polymer's
  * template binding and data observation system.
+ * @template T
+ * @param {function(new:T)} superClass Class to apply mixin to.
+ * @return {function(new:T)} superClass with mixin applied.
  */
 export const PropertyEffects = dedupingMixin(superClass => {
 
@@ -1150,6 +1153,9 @@ export const PropertyEffects = dedupingMixin(superClass => {
       this.__templateInfo;
     }
 
+    /**
+     * @return {!Object<string, string>} Effect prototype property name map.
+     */
     get PROPERTY_EFFECT_TYPES() {
       return TYPES;
     }
diff --git a/lib/mixins/strict-binding-parser.js b/lib/mixins/strict-binding-parser.js
index 924c69c70f..6027fee112 100644
--- a/lib/mixins/strict-binding-parser.js
+++ b/lib/mixins/strict-binding-parser.js
@@ -115,6 +115,9 @@ function storeMethodNumber(bindingData, text, i) {
  * @appliesMixin PropertyEffects
  * @polymer
  * @summary Mixin that parses binding expressions and generates corresponding metadata.
+ * @template T
+ * @param {function(new:T)} superClass Class to apply mixin to.
+ * @return {function(new:T)} superClass with mixin applied.
  */
 const StrictBindingParser = dedupingMixin((base) => {
 
diff --git a/lib/mixins/template-stamp.js b/lib/mixins/template-stamp.js
index 99313a1165..e7a576ec2a 100644
--- a/lib/mixins/template-stamp.js
+++ b/lib/mixins/template-stamp.js
@@ -104,6 +104,9 @@ function createNodeEventHandler(context, eventName, methodName) {
  * @mixinFunction
  * @polymer
  * @summary Element class mixin that provides basic template parsing and stamping
+ * @template T
+ * @param {function(new:T)} superClass Class to apply mixin to.
+ * @return {function(new:T)} superClass with mixin applied.
  */
 export const TemplateStamp = dedupingMixin(
     /**
diff --git a/lib/utils/templatize.js b/lib/utils/templatize.js
index 7d4ed9dd3f..ff488c15d9 100644
--- a/lib/utils/templatize.js
+++ b/lib/utils/templatize.js
@@ -101,7 +101,10 @@ function upgradeTemplate(template, constructor) {
  * @implements {Polymer_PropertyEffects}
  * @private
  */
-const templateInstanceBase = PropertyEffects(class {});
+const templateInstanceBase = PropertyEffects(
+    // This cast shouldn't be neccessary, but Closure doesn't understand that
+    // "class {}" is a constructor function.
+    /** @type {function(new:Object)} */(class {}));
 
 /**
  * @polymer
@@ -317,7 +320,10 @@ TemplateInstanceBase.prototype.__hostProps;
  * @implements {Polymer_MutableData}
  * @private
  */
-const MutableTemplateInstanceBase = MutableData(TemplateInstanceBase);
+const MutableTemplateInstanceBase = MutableData(
+    // This cast shouldn't be necessary, but Closure doesn't seem to understand
+    // this constructor.
+    /** @type {function(new:TemplateInstanceBase)} */(TemplateInstanceBase));
 
 function findMethodHost(template) {
   // Technically this should be the owner of the outermost template.