Fix FileTest that could delete file before a stream on it was closed.

tests/standalone/src/FileTest.dart

Index: tests/standalone/src/FileTest.dart
diff --git a/tests/standalone/src/FileTest.dart b/tests/standalone/src/FileTest.dart
index 9e9f86038a3d56ca7296d92c51055e3f6f21dcee..24da6496943fdf1347da29daaf622f20d3008736 100644
--- a/tests/standalone/src/FileTest.dart
+++ b/tests/standalone/src/FileTest.dart
@@ -771,8 +771,10 @@ class FileTest {
       OutputStream output = file.openOutputStreamSync();
       output.close();
       Expect.throws(() => output.writeFrom(buffer, 0, 12));
-      file.deleteSync();
-      asyncTestDone("testCloseExceptionStream");
+      output.closeHandler = () {
+        file.deleteSync();
+        asyncTestDone("testCloseExceptionStream");
+      };
     };
   }