[ddc] Add types to tearoffs in dart:async patch

These functions were never tagged with types but they do get used
in type tests. Adding the types ensures they work correctly in the
new runtime type system.

Change-Id: I01a2716504072c0b2148ba13167da9814253e9bf
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/265566
Reviewed-by: Mark Zhou <markzipan@google.com>
Commit-Queue: Nicholas Shahan <nshahan@google.com>

sdk/lib/_internal/js_dev_runtime/patch/async_patch.dart

@@ -22,7 +22,6 @@ import 'dart:_runtime' as dart;
 /// Inspired by `co`: https://github.com/tj/co/blob/master/index.js, which is a
 /// stepping stone for ES async/await.
 @JSExportName('async')
-@ReifyFunctionTypes(false)
 _async<T>(Function() initGenerator) {
   var iter;
   late Object? Function(Object?) onValue;