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