Use values.reduce instead of a temporary array

Seems to be faster in v8

lib/utils/html-fn.html

@@ -54,13 +54,9 @@ subject to an additional IP rights grant found at http://polymer.github.io/PATEN
     Polymer.html = function html(strings, ...values) {
       // use raw strings to preserve literal escapes in strings
       const rawStrings = strings.raw;
-      /** @type {!Array<string>} */
-      const parts = [rawStrings[0]];
       const template = /** @type {!HTMLTemplateElement} */(document.createElement('template'));
-      for (let i = 0; i < values.length; i++) {
-        parts.push(htmlValue(values[i]), rawStrings[i + 1]);
-      }
-      template.innerHTML = parts.join('');
+      template.innerHTML = values.reduce((acc, v, idx) =>
+        acc + htmlValue(v) + rawStrings[idx + 1], rawStrings[0]);
       return template;
     };
   })();