Only allow one async operation on RandomAccessFile at a time.

tests/standalone/io/file_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 file I/O.
 
 import 'dart:async';
 import 'dart:convert';
 import 'dart:collection';
 import 'dart:io';
@@ skipping 1064 matching lines @@
 
   static void testLastModified() {
     asyncTestStarted();
     new File(Platform.executable).lastModified().then((modified) {
       Expect.isTrue(modified is DateTime);
       Expect.isTrue(modified.isBefore(new DateTime.now()));
       asyncTestDone("testLastModified");
     });
   }
 
+  static void testDoubleAsyncOperation() {
+    asyncTestStarted();
+    var file = new File(Platform.executable).openSync();
+    var completer = new Completer();
+    int done = 0;
+    bool error = false;
+    void getLength() {
+      file.length()
+          .catchError((e) { error = true; })
+          .whenComplete(() {
+            if (++done == 2) {
+              asyncTestDone("testDoubleAsyncOperation");
+              Expect.isTrue(error);
+              file.lengthSync();
+              file.closeSync();
+            }
+          });
+    }
+    getLength();
+    getLength();
+    Expect.throws(() => file.lengthSync());
+  }
+
   static void testLastModifiedSync() {
     var modified = new File(Platform.executable).lastModifiedSync();
     Expect.isTrue(modified is DateTime);
     Expect.isTrue(modified.isBefore(new DateTime.now()));
   }
 
   // Test that opens the same file for writing then for appending to test
   // that the file is not truncated when opened for appending.
   static void testAppend() {
     asyncTestStarted();
@@ skipping 198 matching lines @@
       testOutputStreamWriteAppend();
       testOutputStreamWriteString();
       testWriteVariousLists();
       testDirectory();
       testDirectorySync();
       testWriteStringUtf8();
       testWriteStringUtf8Sync();
       testRename();
       testRenameSync();
       testLastModified();
+      testDoubleAsyncOperation();
       asyncEnd();
     });
   }
 }
 
 main() {
   FileTest.testMain();
 }