Web: async improvements (#3805)

- Internal: Fix dropping `Notifier` without sending a result causing `Future`s to never complete. This should never happen anyway, but now we get a panic instead of nothing if we hit a bug.
- Internal: Remove a bunch of `unwrap()`s that aren't required when correctly using `MainThreadMarker`.
- `Window::canvas()` is now able to return a reference instead of an owned value.

Extracted from #3801.

src/platform_impl/web/async/wrapper.rs

@@ -33,7 +33,6 @@ unsafe impl<V> Send for Value<V> {}
 unsafe impl<V> Sync for Value<V> {}
 
 impl<V, S: Clone + Send, E> Wrapper<V, S, E> {
-    #[track_caller]
     pub fn new<R: Future<Output = ()>>(
         _: MainThreadMarker,
         value: V,
@@ -41,7 +40,7 @@ impl<V, S: Clone + Send, E> Wrapper<V, S, E> {
         receiver: impl 'static + FnOnce(Arc<RefCell<Option<V>>>) -> R,
         sender_data: S,
         sender_handler: fn(&S, E),
-    ) -> Option<Self> {
+    ) -> Self {
         let value = Arc::new(RefCell::new(Some(value)));
 
         wasm_bindgen_futures::spawn_local({
@@ -52,12 +51,7 @@ impl<V, S: Clone + Send, E> Wrapper<V, S, E> {
             }
         });
 
-        Some(Self {
-            value: Value { value, local: PhantomData },
-            handler,
-            sender_data,
-            sender_handler,
-        })
+        Self { value: Value { value, local: PhantomData }, handler, sender_data, sender_handler }
     }
 
     pub fn send(&self, event: E) {
@@ -68,9 +62,8 @@ impl<V, S: Clone + Send, E> Wrapper<V, S, E> {
         }
     }
 
-    pub fn value(&self) -> Option<Ref<'_, V>> {
-        MainThreadMarker::new()
-            .map(|_| Ref::map(self.value.value.borrow(), |value| value.as_ref().unwrap()))
+    pub fn value(&self, _: MainThreadMarker) -> Ref<'_, V> {
+        Ref::map(self.value.value.borrow(), |value| value.as_ref().unwrap())
     }
 
     pub fn with_sender_data<T>(&self, f: impl FnOnce(&S) -> T) -> T {