Added base template for config ui.

appengine/config_service/ui/bower_components/polymer/externs/webcomponents-externs.js

+/**
+ * @fileoverview Externs for webcomponents polyfills
+ * @externs
+ */
+
+let HTMLImports = {
+  /**
+   * @param {function()} callback
+   */
+  whenReady(callback){}
+};
+
+window.HTMLImports = HTMLImports;
+
+let ShadyDOM = {
+  inUse: false,
+  flush(){},
+  /**
+   * @param {!Node} target
+   * @param {function(Array<MutationRecord>, MutationObserver)} callback
+   * @return {MutationObserver}
+   */
+  observeChildren(target, callback){},
+  /**
+   * @param {MutationObserver} observer
+   */
+  unobserveChildren(observer){},
+  /**
+   * @param {Node} node
+   */
+  patch(node){}
+};
+
+window.ShadyDOM = ShadyDOM;
+
+let WebComponents = {};
+window.WebComponents = WebComponents;
+
+/** @type {Element} */
+HTMLElement.prototype._activeElement;
+
+/**
+ * @param {HTMLTemplateElement} template
+ */
+HTMLTemplateElement.prototype.decorate = function(template){};