Only auto-use disable-upgrade if legacyOptimizations is set.

Author: Steven Orvell

lib/legacy/legacy-element-mixin.js

@@ -21,7 +21,7 @@ import { timeOut, microTask } from '../utils/async.js';
 import { get } from '../utils/path.js';
 import { wrap } from '../utils/wrap.js';
 import { scopeSubtree } from '../utils/scope-subtree.js';
-import { legacyNoObservedAttributes } from '../utils/settings.js';
+import { legacyOptimizations, legacyNoObservedAttributes } from '../utils/settings.js';
 import { findObservedAttributesGetter } from '../mixins/disable-upgrade-mixin.js';
 
 const DISABLED_ATTR = 'disable-upgrade';
@@ -297,7 +297,8 @@ export const LegacyElementMixin = dedupingMixin((base) => {
      */
     _initializeProperties() {
       // NOTE: Inlined for perf from version of DisableUpgradeMixin.
-      if (this.hasAttribute(DISABLED_ATTR)) {
+      // Only auto-use disable-upgrade if legacyOptimizations is set.
+      if (legacyOptimizations && this.hasAttribute(DISABLED_ATTR)) {
         this.__isUpgradeDisabled = true;
       } else {
         let proto = Object.getPrototypeOf(this);