[cfe] Use flatten(T) as future value type for pre-nnbd async functions

Change-Id: I38d537112e66c08bb06bc2493b3126b5f8f797b7
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/239022
Reviewed-by: Chloe Stefantsova <cstefantsova@google.com>
Commit-Queue: Johnni Winther <johnniwinther@google.com>

pkg/kernel/lib/ast.dart

@@ -3673,6 +3673,10 @@ class FunctionNode extends TreeNode {
   /// here the return types are `Future<Foo>` and `FutureOr<Foo>` for `method1`
   /// and `method2`, respectively, but the future value type is in both cases
   /// `Foo`.
+  ///
+  /// For pre-nnbd libraries, this is set to `flatten(T)` of the return type
+  /// `T`, which can be seen as the pre-nnbd equivalent of the future value
+  /// type.
   DartType? futureValueType;
 
   void Function()? lazyBuilder;

pkg/kernel/lib/verifier.dart

@@ -440,7 +440,6 @@ class VerifyingVisitor extends RecursiveResultVisitor<void> {
     AsyncMarker savedAsyncMarker = currentAsyncMarker;
     currentAsyncMarker = node.asyncMarker;
     if (!isOutline &&
-        currentMember!.isNonNullableByDefault &&
         node.asyncMarker == AsyncMarker.Async &&
         node.futureValueType == null) {
       problem(node,