Slightly improve how _registered is called.

This ensures it's not called on extendors who do not specifically implement `_registered`. This is important since it's expected to do prototype specific work.

lib/legacy/class.html

@@ -205,12 +205,8 @@ subject to an additional IP rights grant found at http://polymer.github.io/PATEN
       are controlled via the finalization mechanism provided by `Properties-Mixin`.
 
       `Properties` and `observers` are collected by manually traversing the prototype
-      chain and merging.
-
-      The `_registered` method is called via `LegacyElementMixin._finalizeClass`
-      and is called on each prototype in the element's chain. Because a non-legacy
-      element may extend a legacy one, it's important that work in `_registered`
-      carefully act only once.
+      chain and merging. The `_registered` method is called via
+      `LegacyElementMixin._finalizeClass`.
 
     */
     /**
@@ -224,7 +220,6 @@ subject to an additional IP rights grant found at http://polymer.github.io/PATEN
     function GenerateClassFromInfo(info, Base, behaviors) {
 
       // manages behavior and lifecycle processing (filled in after class definition)
-      let registered = false;
       let activeBehaviors;
       const lifecycle = {};
 
@@ -285,27 +280,20 @@ subject to an additional IP rights grant found at http://polymer.github.io/PATEN
            `is` in `beforeRegister` as you could in 1.x.
           */
           const proto = this;
-          // NOTE: this `registered` flag is required so that extensions
-          // that do not override `_registered` do not try to "re-register"
-          // this data. Only extensions that use `mixinBehaviors` will normally
-          // have this implementation.
-          if (!registered) {
-            registered = true;
-            if (activeBehaviors) {
-              copyAndFilterBehaviors(proto, activeBehaviors, lifecycle);
+          if (activeBehaviors) {
+            copyAndFilterBehaviors(proto, activeBehaviors, lifecycle);
+          }
+          copyAndFilterProperties(proto, info, lifecycle);
+          let list = lifecycle.beforeRegister;
+          if (list) {
+            for (let i=0; i < list.length; i++) {
+              list[i].call(proto);
             }
-            copyAndFilterProperties(proto, info, lifecycle);
-            let list = lifecycle.beforeRegister;
-            if (list) {
-              for (let i=0; i < list.length; i++) {
-                list[i].call(proto);
-              }
-            }
-            list = lifecycle.registered;
-            if (list) {
-              for (let i=0; i < list.length; i++) {
-                list[i].call(proto);
-              }
+          }
+          list = lifecycle.registered;
+          if (list) {
+            for (let i=0; i < list.length; i++) {
+              list[i].call(proto);
             }
           }
         }

lib/legacy/legacy-element-mixin.html

@@ -95,7 +95,12 @@ subject to an additional IP rights grant found at http://polymer.github.io/PATEN
       }
 
       static _finalizeClass() {
-        this.prototype._registered();
+        // Note, call `_registered` only if this specific prototype has
+        // an implementation; this ensures `_registered` is not called
+        // on extenders that do not implement it.
+        if (this.prototype.hasOwnProperty('_registered')) {
+          this.prototype._registered();
+        }
         super._finalizeClass();
       }