Parent property values should come from template. Fixes #2504.

src/lib/template/templatizer.html

@@ -87,6 +87,7 @@
      * @param {HTMLTemplateElement} template The template to process.
      */
     templatize: function(template) {
+      this._templatized = template;
       // TODO(sjmiles): supply _alternate_ content reference missing from root
       // templates (not nested). `_content` exists to provide content sharing
       // for nested templates.
@@ -383,8 +384,9 @@
     stamp: function(model) {
       model = model || {};
       if (this._parentProps) {
+        var templatized = this._templatized;
         for (var prop in this._parentProps) {
-          model[prop] = this[this._parentPropPrefix + prop];
+          model[prop] = templatized[this._parentPropPrefix + prop];
         }
       }
       return new this.ctor(model, this);