Do lazy behavior copying only when legacyOptimizations is set

lib/legacy/class.html

@@ -297,10 +297,10 @@ 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;
-          if (behaviorList) {
-            applyBehaviors(proto, behaviorList, lifecycle);
+          // copy properties lazily if we're optimizing
+          if (Polymer.legacyOptimizations) {
+            copyPropertiesToProto(proto);
           }
-          applyInfo(proto, info, lifecycle, excludeOnInfo);
           let list = lifecycle.beforeRegister;
           if (list) {
             for (let i=0; i < list.length; i++) {
@@ -426,6 +426,18 @@ subject to an additional IP rights grant found at http://polymer.github.io/PATEN
           superBehaviors.concat(behaviors) : behaviorList;
       }
 
+      const copyPropertiesToProto = (proto) => {
+        if (behaviorList) {
+          applyBehaviors(proto, behaviorList, lifecycle);
+        }
+        applyInfo(proto, info, lifecycle, excludeOnInfo);
+      };
+
+      // copy properties if we're not optimizing
+      if (!Polymer.legacyOptimizations) {
+        copyPropertiesToProto(PolymerGenerated.prototype);
+      }
+
       PolymerGenerated.generatedFrom = info;
 
       return PolymerGenerated;