Fix pipe copy test broken by https://dart-review.googlesource.com/c/sdk/+/439823

Change-Id: I8146922de576a080d66959a8aba9192b1c4edf7b
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/439981
Reviewed-by: Alexander Aprelev <aam@google.com>
Commit-Queue: Brian Quinlan <bquinlan@google.com>

tests/standalone/io/named_pipe_operations_test.dart

@@ -86,6 +86,7 @@ main() async {
         final file = File("$stdinPipePath");
         if (file.existsSync()) print("Pipe Exists");
         file.copySync("junk");
+        if (File("junk").existsSync()) print("Pipe Copied!");
       } catch (e) {
         print(e);
       }
@@ -108,7 +109,12 @@ main() async {
     renameScript,
     "Cannot rename file",
   );
-  await startProcess(directory, 'copyscript', copyScript, "Cannot copy file");
+  await startProcess(
+    directory,
+    'copyscript',
+    copyScript,
+    Platform.isMacOS ? "Cannot copy file" : "Pipe Copied!",
+  );
 
   directory.deleteSync(recursive: true);
   asyncEnd();