Behavior property copying fixes

* ensure element has `is` on prototype early as this is sometimes checked in user code.
* ensure properties copied onto elements from info/behaviors are forced to configurable so they can be re-configured by later behaviors.
* add `_noAccessors` optimization for faster property copying

lib/legacy/class.html

@@ -34,7 +34,8 @@ subject to an additional IP rights grant found at http://polymer.github.io/PATEN
       beforeRegister: true,
       registered: true,
       attributeChanged: true,
-      behaviors: true
+      behaviors: true,
+      _noAccessors: true
     };
 
     const excludeOnBehaviors = Object.assign({
@@ -45,13 +46,19 @@ subject to an additional IP rights grant found at http://polymer.github.io/PATEN
     }, excludeOnInfo);
 
     function copyProperties(source, target, excludeProps) {
+      const noAccessors = source._noAccessors;
       for (let p in source) {
-        // NOTE: cannot copy `excludeProps` methods onto prototype at least because
-        // `super.ready` must be called and is not included in the user fn.
         if (!(p in excludeProps)) {
-          let pd = Object.getOwnPropertyDescriptor(source, p);
-          if (pd) {
-            Object.defineProperty(target, p, pd);
+          if (noAccessors) {
+            target[p] = source[p];
+          } else {
+            let pd = Object.getOwnPropertyDescriptor(source, p);
+            if (pd) {
+              // ensure property is configurable so that a later behavior can
+              // re-configure it.
+              pd.configurable = true;
+              Object.defineProperty(target, p, pd);
+            }
           }
         }
       }