[VM/libraries] - Fix signature of waitFor.

This should fix issue 40472.

Bug: 40472
Change-Id: I5cfa7cf25778b6a5430168707dd9370b202d3e6d
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/134565
Reviewed-by: Ryan Macnak <rmacnak@google.com>
Commit-Queue: Siva Annamalai <asiva@google.com>

sdk_nnbd/lib/cli/wait_for.dart

@@ -111,12 +111,12 @@ class _WaitForUtils {
  * subsequent calls block waiting for a condition that is only satisfied when
  * an earlier call returns.
  */
-T? waitFor<T>(Future<T?> future, {Duration? timeout}) {
-  late T? result;
+T waitFor<T>(Future<T> future, {Duration? timeout}) {
+  late T result;
   bool futureCompleted = false;
   Object? error;
   StackTrace? stacktrace;
-  future.then((r) {
+  future.then((T r) {
     futureCompleted = true;
     result = r;
   }, onError: (e, st) {