Add types for Templatizer to Polymer (#27823)

* Add types for Templatizer to Polymer

Polymer was missing the typings for Templatizer, the behavior that
provides helpers for working with template elements in Polymer
components. This adds types for its public interface.

* Add tests for new templatizer types

* Fix modelForElement type to just be Polymer.Base

The closure types here: https://github.com/Polymer/polymer/blob/1.x/src/lib/template/templatizer.html incorrectly suggest that modelForElement returns an `Object<Polymer.Base>`, but in fact it just returns `Polymer.Base`

types/polymer/index.d.ts

@@ -263,7 +263,7 @@ declare global {
       setAttribute(name: string, value: any):void;
 
       removeAttribute(name: string):void;
-        
+
       observeNodes(callback: (info: ObservedNodeInfo) => void): {};
 
       unobserveNodes(observer: {}): void;
@@ -340,6 +340,12 @@ declare global {
       whenLoaded(cb: Function): void;
     }
 
+    interface Templatizer {
+      templatize(template: HTMLTemplateElement, mutableData?: boolean): void;
+      stamp(model: {}): Base;
+      modelForElement: (elem: HTMLElement) => Base;
+    }
+
     interface PolymerStatic {
       Settings: Settings;
 
@@ -351,12 +357,14 @@ declare global {
 
       Class(prototype: Base | { new (): Base }): CustomElementConstructor;
 
-      RenderStatus: RenderStatus
+      RenderStatus: RenderStatus;
 
       ArraySplice: ArraySplice;
 
       /** @deprecated */
-      ImportStatus: ImportStatus
+      ImportStatus: ImportStatus;
+
+      Templatizer: Templatizer;
     }
   }
 

types/polymer/polymer-tests.ts

@@ -1,6 +1,8 @@
 Polymer({
   is: "my-element",
 
+  behaviors: [Polymer.Templatizer],
+
   properties: {
     prop1: String,
     prop2: {
@@ -24,6 +26,11 @@ Polymer({
   },
 
   ready: function () {
+    const template = Polymer.dom(this).querySelector('template');
+    if (template) {
+      this.templatize(template);
+      const instance = this.stamp({item: {}});
+    }
     this.textContent = 'My element!';
     this.$.name.textContent = this.name;
     this.serialize({});