Merge pull request #5520 from Polymer/avoid-template-upgrade

Avoid upgrading template if no hostProps, for better perf.

lib/utils/templatize.js

@@ -360,11 +360,14 @@ function createTemplatizerClass(template, templateInfo, options) {
 }
 
 /**
+ * Adds propagate effects from the template to the template instance for
+ * properties that the host binds to the template using the `_host_` prefix.
+ * 
  * @suppress {missingProperties} class.prototype is not defined for some reason
  */
 function addPropagateEffects(template, templateInfo, options) {
   let userForwardHostProp = options.forwardHostProp;
-  if (userForwardHostProp) {
+  if (userForwardHostProp && templateInfo.hasHostProps) {
     // Provide data API and property effects on memoized template class
     let klass = templateInfo.templatizeTemplateClass;
     if (!klass) {
@@ -423,6 +426,11 @@ function addNotifyEffects(klass, template, templateInfo, options) {
   }
   if (options.forwardHostProp && template.__dataHost) {
     for (let hprop in hostProps) {
+      // As we're iterating hostProps in this function, note whether
+      // there were any, for an optimization in addPropagateEffects
+      if (!templateInfo.hasHostProps) {
+        templateInfo.hasHostProps = true;
+      }
       klass.prototype._addPropertyEffect(hprop,
         klass.prototype.PROPERTY_EFFECT_TYPES.NOTIFY,
         {fn: createNotifyHostPropEffect()});