Add a template setter to ElementMixin.

Currently, to assign a template to a Polymer element, you need to
override the setter, or call Object.defineProperty.

This also allows users to simply assign the template onto their
constructor.

This is especially useful for TypeScript, which compiles "static
template = foo" to "MyClass.template = foo".

lib/mixins/element-mixin.js

@@ -375,7 +375,7 @@ export const ElementMixin = dedupingMixin(base => {
      *     }
      *   }
      *
-     * @return {HTMLTemplateElement|string} Template to be stamped
+     * @return {!HTMLTemplateElement|string} Template to be stamped
      */
     static get template() {
       if (!this.hasOwnProperty(JSCompiler_renameProperty('_template', this))) {
@@ -389,6 +389,15 @@ export const ElementMixin = dedupingMixin(base => {
       return this._template;
     }
 
+    /**
+     * Set the template.
+     *
+     * @param {!HTMLTemplateElement|string} value Template to set.
+     */
+    static set template(value) {
+      this._template = value;
+    }
+
     /**
      * Path matching the url from which the element was imported.
      *