Add node field to PolymerDomApi

And keep DomApiNative type compatible with PolymerDomApi.

Upstreaming cl/275091781

externs/polymer-dom-api-externs.js

@@ -14,7 +14,7 @@
  *
  * @interface
  */
-var PolymerDomApi = function() {};
+let PolymerDomApi = function() {};
 
 /**
  * @param {?Node} node
@@ -103,6 +103,9 @@ PolymerDomApi.prototype.getDestinationInsertionPoints = function() {};
 /** @return {?Node} */
 PolymerDomApi.prototype.getOwnerRoot = function() {};
 
+/** @type {!Node} */
+PolymerDomApi.prototype.node;
+
 /**
  * @param {string} attribute
  * @param {string} value

lib/legacy/polymer.dom.js

@@ -45,7 +45,7 @@ export const matchesSelector = function(node, selector) {
 class DomApiNative {
 
   /**
-   * @param {Node} node Node for which to create a Polymer.dom helper object.
+   * @param {!Node} node Node for which to create a Polymer.dom helper object.
    */
   constructor(node) {
     if (window['ShadyDOM'] && window['ShadyDOM']['inUse']) {