Improve types for the template field on Polymer elements. (#5596)

null is allowed!

externs/polymer-externs.js

@@ -41,7 +41,7 @@ PolymerInit.prototype.extends;
 PolymerInit.prototype.properties;
 /** @type {(!Array<string> | undefined)} */
 PolymerInit.prototype.observers;
-/** @type {(!HTMLTemplateElement | string | undefined)} */
+/** @type {(!HTMLTemplateElement | string | undefined | null)} */
 PolymerInit.prototype.template;
 /** @type {(!Object<string, *> | undefined)} */
 PolymerInit.prototype.hostAttributes;
@@ -60,7 +60,7 @@ PolymerElementConstructor.extends;
 PolymerElementConstructor.properties;
 /** @type {(!Array<string> | undefined)} */
 PolymerElementConstructor.observers;
-/** @type {(!HTMLTemplateElement | string | undefined)} */
+/** @type {(!HTMLTemplateElement | string | undefined | null)} */
 PolymerElementConstructor.template;
 
 /** @interface */
@@ -150,7 +150,7 @@ var PolymerElement = function() {};
 PolymerElement.is;
 /**
  * The template to stamp when creating this element type.
- * @type {!HTMLTemplateElement|undefined}
+ * @type {!HTMLTemplateElement|undefined|null}
  */
 PolymerElement.template;
 /**

lib/elements/dom-bind.js

@@ -115,7 +115,7 @@ export class DomBind extends domBindBase {
   render() {
     let template;
     if (!this.__children) {
-      template = /** @type {HTMLTemplateElement} */(template || this.querySelector('template'));
+      template = /** @type {?HTMLTemplateElement} */(template || this.querySelector('template'));
       if (!template) {
         // Wait until childList changes and template should be there by then
         let observer = new MutationObserver(() => {