Loosen _attachDom return signature.

CHANGELOG.md

@@ -1,5 +1,8 @@
 # Change Log
 
+## [UNRELEASED]() ()
+- Loosen _attachDom return signature. ([commit]())
+
 ## [v3.4.1](https://github.com/Polymer/polymer/tree/v3.4.1) (2020-04-29)
 - [ci skip] bump to 3.4.1 ([commit](https://github.com/Polymer/polymer/commit/12fa1b50))
 

lib/mixins/element-mixin.d.ts

@@ -218,7 +218,7 @@ interface ElementMixin extends PropertyEffects, TemplateStamp, PropertyAccessors
    * @param dom to attach to the element.
    * @returns node to which the dom has been attached.
    */
-  _attachDom(dom: StampedTemplate|null): ShadowRoot|null;
+  _attachDom(dom: StampedTemplate|null): Node|null;
 
   /**
    * When using the ShadyCSS scoping and custom property shim, causes all

lib/mixins/element-mixin.js

@@ -731,7 +731,7 @@ export const ElementMixin = dedupingMixin(base => {
      * @throws {Error}
      * @suppress {missingReturn}
      * @param {StampedTemplate} dom to attach to the element.
-     * @return {ShadowRoot} node to which the dom has been attached.
+     * @return {Node} node to which the dom has been attached.
      */
     _attachDom(dom) {
       const n = wrap(this);

test/unit/mixin-utils.html

@@ -117,4 +117,4 @@
 });
 </script>
 </body>
-</html>
+</html>

test/unit/polymer.element.html

@@ -80,6 +80,11 @@
     this._calledReady++;
   }
 
+  _attachDom(dom) {
+    this.appendChild(dom);
+    return this;
+  }
+
   _compute(value) {
     return value;
   }
@@ -401,6 +406,7 @@ <h1>Sub template</h1>
     assert.equal(el.prop, 'base');
     assert.equal(el.computedPropDep, true);
     assert.equal(el.computedProp, true);
+    assert.isNull(el.shadowRoot);
   });
 
   test('attributes', function() {