Fix flaking file lock test

tests/standalone/io/file_blocking_lock_test.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.
 
 // This test works by spawning a new process running
 // file_blocking_lock_script.dart, trading the file lock back and forth,
 // writing bytes 1 ... 25 in order to the file. There are checks to ensure
 // that the bytes are written in order, that one process doesn't write all the
 // bytes and that a non-blocking lock fails such that a blocking lock must
 // be taken, which succeeds.
@@ skipping 21 matching lines @@
         .listen((data) { print(data); });
     process.stderr
         .transform(UTF8.decoder)
         .listen((data) { print(data); });
     return process;
   });
 }
 
 testLockWholeFile() async {
   const int length = 25;
-  asyncStart();
   Directory directory = await Directory.systemTemp.createTemp('dart_file_lock');
   File file = new File(join(directory.path, "file"));
   await file.writeAsBytes(new List.filled(length, 0));
   var raf = await file.open(mode: APPEND);
   await raf.setPosition(0);
   await raf.lock(FileLock.BLOCKING_EXCLUSIVE, 0, length);
   Process peer = await runPeer(file.path, length, FileLock.BLOCKING_EXCLUSIVE);
 
-  // Wait a bit for the other process to get started. We'll synchronize on
-  // the file lock.
-  await new Future.delayed(const Duration(seconds: 1));
-
   int nextToWrite = 1;
   int at = 0;
   List iWrote = new List.filled(length, 0);
   bool nonBlockingFailed = false;
   while (nextToWrite <= length) {
     int p = await raf.position();
     await raf.writeByte(nextToWrite);
     await raf.flush();
     // Record which bytes this process wrote so that we can check that the
     // other process was able to take the lock and write some bytes.
     iWrote[nextToWrite-1] = nextToWrite;
     nextToWrite++;
-    await raf.unlock(0, length);
-    try {
-      await raf.lock(FileLock.EXCLUSIVE, 0, length);
-    } catch(e) {
-      // Check that at some point the non-blocking lock fails.
-      nonBlockingFailed = true;
-      await raf.lock(FileLock.BLOCKING_EXCLUSIVE, 0, length);
+    // Let the other process get the lock at least once by spinning until the
+    // non-blocking lock fails.
+    while (!nonBlockingFailed) {
+      await raf.unlock(0, length);
+      try {
+        await raf.lock(FileLock.EXCLUSIVE, 0, length);
+      } catch(e) {
+        // Check that at some point the non-blocking lock fails.
+        nonBlockingFailed = true;
+        await raf.lock(FileLock.BLOCKING_EXCLUSIVE, 0, length);
+      }
     }
     while (true) {
       p = await raf.position();
       at = await raf.readByte();
       if (at == 0 || at == -1) break;
       nextToWrite++;
     }
     await raf.setPosition(p);
   }
 
   await raf.setPosition(0);
   for (int i = 1; i <= length; i++) {
     Expect.equals(i, await raf.readByte());
   }
   await raf.unlock(0, length);
 
   bool wroteAll = true;
   for (int i = 0; i < length; i++) {
     // If there's a 0 entry, this process didn't write all bytes.
     wroteAll = wroteAll && (iWrote[i] == 0);
   }
   Expect.equals(false, wroteAll);
 
   Expect.equals(true, nonBlockingFailed);
 
-  peer.exitCode.then((v) {
+  await peer.exitCode.then((v) async {
     Expect.equals(0, v);
-    raf.closeSync();
-    directory.deleteSync(recursive: true);
-    asyncEnd();
+    await raf.close();
+    await directory.delete(recursive: true);
   });
 }
 
-main() {
-  testLockWholeFile();
+main() async {
+  asyncStart();
+  await testLockWholeFile();
+  asyncEnd();
 }