Minor fixes

* [PropertiesChanged]: check for and call `super.attributeChangedCallback`
* [PropertiesMixin]:
  * remove `disconnectedCallback` stub.
  * Improve comments

lib/mixins/properties-changed.html

@@ -369,11 +369,15 @@
          * @param {string} name Name of attribute that changed
          * @param {?string} old Old attribute value
          * @param {?string} value New attribute value
+         * @suppress {missingProperties} Super may or may not implement the callback
          */
         attributeChangedCallback(name, old, value) {
           if (old !== value) {
             this._attributeToProperty(name, value);
           }
+          if (super.attributeChangedCallback) {
+            super.attributeChangedCallback(name, old, value);
+          }
         }
 
         /**

lib/mixins/properties-mixin.html

@@ -17,15 +17,15 @@
   'use strict';
 
   /**
-    * Mixes `moreProps` into `props` but upgrades shorthand type
-    * syntax to { type: Type}.
-    *
-    * @param {Object} props Properties to normalize
-    * @return {Object} Copy of input `props` with normalized properties that
-    * are in the form {type: Type}
-    * @private
-    */
-    function normalizeProperties(props) {
+   * Creates a copy of `props` with each property normalized such that
+   * upgraded it is an object with at least a type property { type: Type}.
+   *
+   * @param {Object} props Properties to normalize
+   * @return {Object} Copy of input `props` with normalized properties that
+   * are in the form {type: Type}
+   * @private
+   */
+  function normalizeProperties(props) {
     const output = {};
     for (let p in props) {
       const o = props[p];
@@ -74,7 +74,7 @@
       // generated by this call to the mixin; the instanceof test only works
       // because the mixin is deduped and guaranteed only to apply once, hence
       // all constructors in a proto chain will see the same `PropertiesMixin`
-      return (superCtor.prototype instanceof PropertiesMixin) ? 
+      return (superCtor.prototype instanceof PropertiesMixin) ?
         /** @type {PropertiesMixinConstructor} */ (superCtor) : null;
     }
 
@@ -120,7 +120,7 @@
        * accessors exist on the element prototype. This method calls
        * `_finalizeClass` to finalize each constructor in the prototype chain.
        */
-       static finalize() {
+      static finalize() {
         if (!this.hasOwnProperty(JSCompiler_renameProperty('__finalized', this))) {
           const superCtor = superPropertiesClass(/** @type {PropertiesMixinConstructor} */(this));
           if (superCtor) {
@@ -200,16 +200,6 @@
         this._enableProperties();
       }
 
-      /**
-       * Called when the element is removed from a document
-       * @suppress {missingProperties} Super may or may not implement the callback
-       */
-       disconnectedCallback() {
-        if (super.disconnectedCallback) {
-          super.disconnectedCallback();
-        }
-       }
-
     }
 
     return PropertiesMixin;