Make sure to close file in test so the temp directory can be deleted on Windows

tests/standalone/io/file_error_test.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 //
 // Dart test program for testing error handling in file I/O.
 
 import "dart:io";
 import "dart:isolate";
 
 Directory tempDir() {
@@ skipping 441 matching lines @@
     port.send(null);
   });
 }
 
 testReadSyncBigInt() {
   createTestFile((file, port) {
     var bigint = 100000000000000000000000000000000000000000;
     var openedFile = file.openSync();
     Expect.throws(() => openedFile.readSync(bigint),
                   (e) => e is FileIOException);
+    openedFile.closeSync();
     port.send(null);
   });
 }
 
 testReadSyncClosedFile() {
   createTestFile((file, port) {
     var openedFile = file.openSync();
     openedFile.closeSync();
     Expect.throws(() => openedFile.readSync(1),
                   (e) => e is FileIOException);
@@ skipping 13 matching lines @@
   testReadAsLinesNonExistent();
   testWriteByteToReadOnlyFile();
   testWriteListToReadOnlyFile();
   testTruncateReadOnlyFile();
   testOperateOnClosedFile();
   testRepeatedlyCloseFile();
   testRepeatedlyCloseFileSync();
   testReadSyncBigInt();
   testReadSyncClosedFile();
 }