Fix newly added regression test that left processes hanging on Windows.

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 81f9eda664bdfa25802219f3cb16d655ead3d295..75284be271da0af23e528b723eb04d9b6b822e4a 100644
--- a/tests/standalone/io/regress_7191_test.dart
+++ b/tests/standalone/io/regress_7191_test.dart
@@ -10,8 +10,10 @@
 // closed which will make this test hang.
 
 import 'dart:io';
+import 'dart:isolate';
 
 main() {
+  var port = new ReceivePort();
   var options = new Options();
   var executable = options.executable;
   var scriptDir = new Path.fromNative(options.script).directoryPath;
@@ -23,5 +25,6 @@ main() {
     process.stdout.onClosed = () {
       process.stdin.write([0]);
     };
+    process.onExit = (exitCode) => port.close();
   });
-}
+}