Fixes leak of native File objects.

tests/standalone/io/file_lock_test.dart

 // Copyright (c) 2015, 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.
 
 import 'dart:async';
 import 'dart:io';
 
 import "package:async_helper/async_helper.dart";
 import "package:expect/expect.dart";
 import "package:path/path.dart";
@@ skipping 18 matching lines @@
             print("  stderr:");
             print(result.stderr);
             print("  arguments:");
             print(arguments);
             Expect.fail('Client subprocess exit code: ${result.exitCode}');
           }
    });
 }
 
 checkLocked(String path,
-            [int start, int end, FileLock mode = FileLock.EXCLUSIVE]) =>
+    [int start = 0, int end = -1, FileLock mode = FileLock.EXCLUSIVE]) =>
     check(path, start, end, mode, locked: true);
 
 checkNotLocked(String path,
-               [int start, int end, FileLock mode = FileLock.EXCLUSIVE]) =>
+    [int start = 0, int end = -1, FileLock mode = FileLock.EXCLUSIVE]) =>
     check(path, start, end, mode, locked: false);
 
 void testLockWholeFile() {
   Directory directory = Directory.systemTemp.createTempSync('dart_file_lock');
   File file = new File(join(directory.path, "file"));
   file.writeAsBytesSync(new List.filled(10, 0));
   var raf = file.openSync(mode: WRITE);
   raf.lockSync();
   asyncStart();
   checkLocked(file.path).then((_) {
@@ skipping 262 matching lines @@
   testLockWholeFileAsync();
   testLockRange();
   testLockRangeAsync();
   testLockEnd();
   testLockEndAsync();
   testLockShared();
   testLockSharedAsync();
   testLockAfterLength();
   testLockAfterLengthAsync();
 }