Make it an error to call close twice on a file using the async API.

tests/standalone/src/io/FileInvalidArgumentsTest.dart

 // Copyright (c) 2012, 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:io");
 
 class FileTest {
   static void testOpenInvalidArgs(name) {
     var file = new File(name);
     try {
@@ skipping 48 matching lines @@
       file.readListSync(buffer, offset, length);
       Expect.fail('exception expected');
     } catch (var e) {
       Expect.isTrue(e is FileIOException);
       Expect.isTrue(e.toString().contains('Invalid arguments'));
     }
 
     var errors = 0;
     file.onError = (s) {
       errors++;
-      Expect.isTrue(s.contains('Invalid arguments'));
+      Expect.isTrue(s is FileIOException);
+      Expect.isTrue(s.toString().contains('Invalid arguments'));
       file.close(() {
         Expect.equals(1, errors);
       });
     };
     file.readList(buffer, offset, length, (bytes) {
       Expect.fail('read list invalid arguments');
     });
   }
 
   static void testWriteByteInvalidArgs(value) {
     String filename = getFilename("tests/vm/data/fixed_length_file");
     var file = (new File(filename + "_out")).openSync(FileMode.WRITE);
     try {
       file.writeByteSync(value);
       Expect.fail('exception expected');
     } catch (var e) {
       Expect.isTrue(e is FileIOException);
       Expect.isTrue(e.toString().contains('Invalid argument'));
     }
 
     var errors = 0;
     file.onError = (s) {
       errors++;
-      Expect.isTrue(s.contains('Invalid argument'));
+      Expect.isTrue(s is FileIOException);
+      Expect.isTrue(s.toString().contains('Invalid argument'));
       file.close(() {
         Expect.equals(1, errors);
       });
     };
     var calls = 0;
     file.writeByte(value);
     file.onNoPendingWrites = () {
       if (++calls > 1) Expect.fail('write list invalid argument');
     };
   }
 
   static void testWriteListInvalidArgs(buffer, offset, bytes) {
     String filename = getFilename("tests/vm/data/fixed_length_file");
     var file = (new File(filename + "_out")).openSync(FileMode.WRITE);
     try {
       file.writeListSync(buffer, offset, bytes);
       Expect.fail('exception expected');
     } catch (var e) {
       Expect.isTrue(e is FileIOException);
       Expect.isTrue(e.toString().contains('Invalid arguments'));
     }
 
     var errors = 0;
     file.onError = (s) {
       errors++;
-      Expect.isTrue(s.contains('Invalid arguments'));
+      Expect.isTrue(s is FileIOException);
+      Expect.isTrue(s.toString().contains('Invalid arguments'));
       file.close(() {
         Expect.equals(1, errors);
       });
     };
     var calls = 0;
     file.writeList(buffer, offset, bytes);
     file.onNoPendingWrites = () {
       if (++calls > 1) Expect.fail('write string invalid argument');
     };
   }
@@ skipping 53 matching lines @@
   FileTest.testCreateInvalidArgs(0);
   FileTest.testReadListInvalidArgs(12, 0, 1);
   FileTest.testReadListInvalidArgs(new List(10), '0', 1);
   FileTest.testReadListInvalidArgs(new List(10), 0, '1');
   FileTest.testWriteByteInvalidArgs('asdf');
   FileTest.testWriteListInvalidArgs(12, 0, 1);
   FileTest.testWriteListInvalidArgs(new List(10), '0', 1);
   FileTest.testWriteListInvalidArgs(new List(10), 0, '1');
   FileTest.testFullPathInvalidArgs(12);
 }