diff --git a/lib/legacy/class.js b/lib/legacy/class.js
index 87ac046c73..dd178dcde0 100644
--- a/lib/legacy/class.js
+++ b/lib/legacy/class.js
@@ -10,7 +10,7 @@ subject to an additional IP rights grant found at http://polymer.github.io/PATEN
 
 import { LegacyElementMixin } from './legacy-element-mixin.js';
 import { legacyOptimizations } from '../utils/settings.js';
-import { wrap } from '../utils/wrap.js';
+import { DisableUpgradeMixin } from '../mixins/disable-upgrade-mixin.js';
 
 const lifecycleProps = {
   attached: true,
@@ -176,10 +176,6 @@ function mergeProperties(target, source) {
   }
 }
 
-const DISABLED_ATTR = 'disable-upgrade';
-
-let observedAttributesGetter;
-
 /* Note about construction and extension of legacy classes.
   [Changed in Q4 2018 to optimize performance.]
 
@@ -216,97 +212,9 @@ function GenerateClassFromInfo(info, Base, behaviors) {
   let behaviorList;
   const lifecycle = {};
 
-  // Work around for closure bug #126934458. Using `super` in a property
-  // getter does not work so instead we search the Base prototype for an
-  // implementation of observedAttributes so that we can override and call
-  // the `super` getter. Note, this is done one time ever because we assume
-  // that `Base` is always comes from `Polymer.LegacyElementMixn`.
-  if (!observedAttributesGetter) {
-    let ctor = Base;
-    while (ctor && !observedAttributesGetter) {
-      const desc = Object.getOwnPropertyDescriptor(ctor, 'observedAttributes');
-      if (desc) {
-        observedAttributesGetter = desc.get;
-      }
-      ctor = Object.getPrototypeOf(ctor.prototype).constructor;
-    }
-  }
-
   /** @private */
   class PolymerGenerated extends Base {
 
-    // NOTE, this copies most of the code from DisableUpgradeMixin. This is
-    // unfortunate, but testing revealed a small penalty to applying the mixin
-    // on top of every generated class, so we do it directly here.
-    static get observedAttributes() {
-      return observedAttributesGetter.call(this).concat(DISABLED_ATTR);
-    }
-
-    // Prevent element from initializing properties when it's upgrade disabled.
-    /** @override */
-    _initializeProperties() {
-      // Enable disable-upgrade use when `legacyOptimizations` setting is on.
-      if (legacyOptimizations && this.hasAttribute(DISABLED_ATTR)) {
-        this.__isUpgradeDisabled = true;
-      } else {
-        super._initializeProperties();
-      }
-    }
-
-    // Prevent element from enabling properties when it's upgrade disabled.
-    // Normally overriding connectedCallback would be enough, but dom-* elements
-    // enable elements early.
-    /** @override */
-    _enableProperties() {
-      if (!this.__isUpgradeDisabled) {
-        super._enableProperties();
-      }
-    }
-
-    /**
-     * @override
-     * @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 {void}
-     */
-    attributeChangedCallback(name, old, value, namespace) {
-      if (name == DISABLED_ATTR) {
-        // When disable-upgrade is removed, intialize properties and
-        // provoke connectedCallback if the element is already connected.
-        if (this.__isUpgradeDisabled && value == null) {
-          super._initializeProperties();
-          this.__isUpgradeDisabled = false;
-          if (wrap(this).isConnected) {
-            super.connectedCallback();
-          }
-        }
-      } else {
-        super.attributeChangedCallback(
-            name, old, value, /** @type {null|string} */ (namespace));
-      }
-    }
-
-    // Prevent element from connecting when it's upgrade disabled.
-    // This prevents user code in `attached` from being called.
-    /** @override */
-    connectedCallback() {
-      if (!this.__isUpgradeDisabled) {
-        super.connectedCallback();
-      }
-    }
-
-    // Prevent element from disconnecting when it's upgrade disabled.
-    // This avoids allowing user code `detached` from being called without a
-    // paired call to `attached`.
-    /** @override */
-    disconnectedCallback() {
-      if (!this.__isUpgradeDisabled) {
-        super.disconnectedCallback();
-      }
-    }
-
     // explicitly not calling super._finalizeClass
     /** @nocollapse */
     static _finalizeClass() {
@@ -626,6 +534,9 @@ export const Class = function(info, mixin) {
   let klass = mixin ? mixin(LegacyElementMixin(HTMLElement)) :
       LegacyElementMixin(HTMLElement);
   klass = GenerateClassFromInfo(info, klass, info.behaviors);
+  if (legacyOptimizations) {
+    klass = DisableUpgradeMixin(klass);
+  }
   // decorate klass with registration info
   klass.is = klass.prototype.is = info.is;
   return klass;