[dart2wasm] Fix awaiting non-future values

New passing tests:

- co19/Language/Statements/For/Asynchronous_For_in/execution_A05_t05
- language/async/await_test/02
- language/async/await_test/03
- language/async/await_test/none
- language/await/nonfuture_test
- language/await/started_immediately_test

Change-Id: I5d5e13f846aa2415abb9788ef341b1d86170937c
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/286921
Reviewed-by: Aske Simon Christensen <askesc@google.com>
Commit-Queue: Ömer Ağacan <omersa@google.com>

sdk/lib/_internal/wasm/lib/async_patch.dart

@@ -95,7 +95,9 @@ Object? _awaitHelper(Object? operand, WasmExternRef? stack) {
   // ensure that the zone will be restored in the event of an exception by
   // restoring the original zone before we throw the exception.
   _Zone current = Zone._current;
-  if (operand is! Future) return operand;
+  if (operand is! Future) {
+    operand = Future.value(operand);
+  }
   Object? result = _futurePromise(stack, operand);
   Zone._leave(current);
   if (result is _FutureError) {