Adds blocking file locks.

tests/standalone/io/file_blocking_lock_script.dart

+// Copyright (c) 2016, 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.
+//
+// Script used by the file_lock_test.dart test.
+
+import "dart:async";
+import "dart:io";
+
+Future<int> testLockWholeFile(File file, int len) async {
+  var raf = await file.open(mode: APPEND);
+  await raf.setPosition(0);
+  int nextToWrite = 1;
+  while (nextToWrite <= len) {
+    await raf.lock(FileLock.BLOCKING_EXCLUSIVE, 0, len);
+
+    int at;
+    int p;
+    while (true) {
+      p = await raf.position();
+      at = await raf.readByte();
+      if (at == 0 || at == -1) break;
+      nextToWrite++;
+    }
+    await raf.setPosition(p);
+    await raf.writeByte(nextToWrite);
+    await raf.flush();
+    nextToWrite++;
+    await raf.unlock(0, len);
+  }
+
+  await raf.lock(FileLock.BLOCKING_EXCLUSIVE, 0, len);
+  await raf.setPosition(0);
+  for (int i = 1; i <= len; i++) {
+    if ((await raf.readByte()) != i) {
+      await raf.unlock(0, len);
+      await raf.close();
+      return 1;
+    }
+  }
+  await raf.unlock(0, len);
+  await raf.close();
+  return 0;
+}
+
+main(List<String> args) async {
+  File file = new File(args[0]);
+  int len = int.parse(args[1]);
+  exit(await testLockWholeFile(file, len));
+}