Address feedback

- Only support HTMLTemplateElement and strings
- Use raw strings for the constants to handle single `\`

lib/utils/html-fn.html

@@ -17,18 +17,18 @@ subject to an additional IP rights grant found at http://polymer.github.io/PATEN
      * @return {string} HTML stringified form of `obj`
      */
     function htmlValue(value) {
-      if (value instanceof HTMLElement) {
-        return /** @type {!HTMLElement} */(value).innerHTML;
-      } else if (value instanceof Node) {
-        return /** @type {!Node} */(value).textContent;
+      if (value === null || value === undefined) {
+        return '';
+      } else if (value instanceof HTMLTemplateElement) {
+        return /** @type {!HTMLTemplateElement} */(value).innerHTML;
       } else {
-        return value.toString();
+        return String(value);
       }
     }
 
     /**
      * Small tagged template string function to facilitate building a `<template>` element.
-     * Polymer.html will automatically compose HTML elements in `${}` blocks, allowing for easy
+     * Polymer.html will automatically compose `<template>` elements in `${}` blocks, allowing for easy
      * composition of superclass templates, or partial templates.
      *
      * Example:
@@ -44,15 +44,17 @@ subject to an additional IP rights grant found at http://polymer.github.io/PATEN
      *
      * @memberof Polymer
      * @param {!Array<string>} strings Constant parts of tagged template literal
-     * @param {!Array<*>} values Variable parts of tagged template literal
+     * @param {...*} values Variable parts of tagged template literal
      * @return {!HTMLTemplateElement} Constructed HTMLTemplateElement
      */
     Polymer.html = function html(strings, ...values) {
+      // use raw strings to preserve literal escapes in strings
+      const rawStrings = strings.raw;
       /** @type {!Array<string>} */
-      const parts = [strings[0]];
+      const parts = [rawStrings[0]];
       const template = /** @type {!HTMLTemplateElement} */(document.createElement('template'));
       for (let i = 0; i < values.length; i++) {
-        parts.push(htmlValue(values[i]), strings[i + 1]);
+        parts.push(htmlValue(values[i]), rawStrings[i + 1]);
       }
       template.innerHTML = parts.join('');
       return template;

test/smoke/html-fn.html

@@ -38,6 +38,7 @@ subject to an additional IP rights grant found at http://polymer.github.io/PATEN
         return html`
           <style>.frame {font-style: italic}</style>
           <div class="frame">${super.template}</div>
+          <div>\</div>
         `;
       }
       constructor() {