Copy flag to a single location rather than two.

lib/legacy/legacy-element-mixin.js

@@ -168,8 +168,7 @@ export const LegacyElementMixin = dedupingMixin((base) => {
 
     // NOTE: Inlined for perf from version of DisableUpgradeMixin.
     static get observedAttributes() {
-      if (legacyNoObservedAttributes && !this._legacyForceObservedAttributes) {
-        this.prototype._legacyForceObservedAttributes = this._legacyForceObservedAttributes;
+      if (legacyNoObservedAttributes && !this.prototype._legacyForceObservedAttributes) {
         // Ensure this element is property registered with the telemetry system.
         if (!this.hasOwnProperty(JSCompiler_renameProperty('__observedAttributes', this))) {
           this.__observedAttributes = [];

lib/legacy/polymer-fn.js

@@ -41,7 +41,7 @@ const Polymer = function(info) {
   }
   // Copy opt out for `legacyNoObservedAttributes` from info object to class.
   if (info._legacyForceObservedAttributes) {
-    klass._legacyForceObservedAttributes = info._legacyForceObservedAttributes;
+    klass.prototype._legacyForceObservedAttributes = info._legacyForceObservedAttributes;
   }
   customElements.define(klass.is, /** @type {!HTMLElement} */(klass));
   return klass;