webidl: All interfaces implicitly extend Object

This information is embedded within the algorithm for constructing interfaces
and their prototypes in the section for ECMAScript glue in the WebIDL spec...

This really *should* make the `wasm_bindgen_backend::ast::ImportType::extends`
member from a `Vec<Ident>` into a `Vec<syn::Path>` so that we could use
`js_sys::Object` in the extends field, but that is a huge pain because then the
`ImportedTypes` trait needs to be changed, and all of its implementers, etc...

crates/web-sys/tests/wasm/event.rs

@@ -1,6 +1,7 @@
 use futures::future::Future;
-use js_sys::Promise;
+use js_sys::{Object, Promise};
 use wasm_bindgen::prelude::*;
+use wasm_bindgen::JsCast;
 use wasm_bindgen_futures::JsFuture;
 use wasm_bindgen_test::*;
 use web_sys::Event;
@@ -15,6 +16,10 @@ fn event() -> impl Future<Item = (), Error = JsValue> {
     JsFuture::from(new_event())
         .map(Event::from)
         .map(|event| {
+            // All DOM interfaces should inherit from `Object`.
+            assert!(event.is_instance_of::<Object>());
+            let _: &Object = event.as_ref();
+
             // These should match `new Event`.
             assert!(event.bubbles());
             assert!(event.cancelable());