Fix flaking file lock test

tests/standalone/io/file_blocking_lock_test.dart

Index: tests/standalone/io/file_blocking_lock_test.dart
diff --git a/tests/standalone/io/file_blocking_lock_test.dart b/tests/standalone/io/file_blocking_lock_test.dart
index ef826a2037075efa3dd2f35f13fd8db7653dff36..7445cb85405e7fb2257afa71d91911bffa9e7795 100644
--- a/tests/standalone/io/file_blocking_lock_test.dart
+++ b/tests/standalone/io/file_blocking_lock_test.dart
@@ -39,7 +39,6 @@ runPeer(String path, int len, FileLock mode) {
 
 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));
@@ -64,13 +63,17 @@ testLockWholeFile() async {
     // 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();
@@ -96,14 +99,15 @@ testLockWholeFile() async {
 
   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();
 }