[vm] Fix windows build after 722b839.

Change-Id: Ie6eed25ae159deaf60852d29bca4e35c0084b1ba
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/107403
Reviewed-by: Samir Jindel <[email protected]>

Author: sjindel-google

runtime/vm/dart_api_impl_test.cc

@@ -82,7 +82,7 @@ UNIT_TEST_CASE(DartAPI_DartInitializeCallsCodeObserver) {
 
   // Wait for 5 seconds to let the kernel service load the snapshot,
   // which should trigger calls to the code observer.
-  sleep(5);
+  OS::Sleep(5);
 
   EXPECT(was_called);
   EXPECT(Dart_Cleanup() == NULL);