Remove connected callback externs and typecast instead

src/polyfills/custom-elements.extern.js

@@ -18,12 +18,3 @@
 // window externs file.
 /** @type {!typeof HTMLElement} */
 window.HTMLElementOrig;
-
-// These callbacks are used for custom elements, but don't appear to belong to
-// any available type definitions. There's a TODO(jridgewell) from ~2 years ago
-// to call them differently; if that can be figured out, these may be dropped.
-// If not, these may belong to a separate node.extern.js externs file.
-/** @function @this {Node} */
-Node.prototype.connectedCallback;
-/** @function @this {Node} */
-Node.prototype.disconnectedCallback;

src/polyfills/custom-elements.js

@@ -424,7 +424,8 @@ class Registry {
     if (!def) {
       return;
     }
-    this.upgradeSelf_(/** @type {!Element} */ (node), def);
+    node = /** @type {!HTMLElement} */ (node);
+    this.upgradeSelf_(node, def);
     // TODO(jridgewell): It may be appropriate to adoptCallback, if the node
     // used to be in another doc.
     // TODO(jridgewell): I should be calling the definitions connectedCallback
@@ -446,6 +447,7 @@ class Registry {
   disconnectedCallback_(node) {
     // TODO(jridgewell): I should be calling the definitions connectedCallback
     // with node as the context.
+    node = /** @type {!HTMLElement} */ (node);
     if (node.disconnectedCallback) {
       try {
         node.disconnectedCallback();