Validate exit-code in regress_7191.

tests/standalone/io/regress_7191_test.dart

Index: tests/standalone/io/regress_7191_test.dart
diff --git a/tests/standalone/io/regress_7191_test.dart b/tests/standalone/io/regress_7191_test.dart
index 6ce1e36b8a598a689f6c9f7c96a7ce337ce123a5..0262f4d1942448e4d95166283ec050c1c5774642 100644
--- a/tests/standalone/io/regress_7191_test.dart
+++ b/tests/standalone/io/regress_7191_test.dart
@@ -23,6 +23,9 @@ main() {
     process.stdout.listen((_) { },
                            onDone: () { process.stdin.writeBytes([0]); });
     process.stderr.listen((_) { });
-    process.exitCode.then((exitCode) => port.close());
+    process.exitCode.then((exitCode) {
+      port.close();
+      if (exitCode != 0) throw "Bad exit code";
+    });
   });
 }