Remove check whether async has been used from File objects.

tests/standalone/src/io/FileTest.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.
 //
 // Dart test program for testing file I/O.
 
 #import("dart:io");
 #import("dart:isolate");
 
 class MyListOfOneElement implements List {
@@ skipping 818 matching lines @@
       exceptionCaught = true;
     } catch (Exception ex) {
       wrongExceptionCaught = true;
     }
     Expect.equals(true, exceptionCaught);
     Expect.equals(true, !wrongExceptionCaught);
     openedFile.closeSync();
     file.deleteSync();
   }
 
-  static void testMixedSyncAndAsync() {
-    var name = getFilename("tests/vm/data/fixed_length_file");
-    var f = new File(name);
-    f.onError = (e) => Expect.fail("No errors expected: $e");
-    f.exists((exists) {
-      try {
-        f.existsSync();
-        Expect.fail("Expected exception");
-      } catch (var e) {
-        Expect.isTrue(e is FileIOException);
-      }
-    });
-  }
-
   static void testOpenDirectoryAsFile() {
     var f = new File('.');
     f.open(FileMode.READ, (r) => Expect.fail('Directory opened as file'));
   }
 
   static void testOpenDirectoryAsFileSync() {
     var f = new File('.');
     try {
       f.openSync();
       Expect.fail("Expected exception opening directory as file");
@@ skipping 186 matching lines @@
 
   // Main test entrypoint.
   static testMain() {
     testRead();
     testReadSync();
     testReadStream();
     testLength();
     testLengthSync();
     testPosition();
     testPositionSync();
-    testMixedSyncAndAsync();
     testOpenDirectoryAsFile();
     testOpenDirectoryAsFileSync();
     testReadAsBytes();
     testReadAsBytesSync();
     testReadAsText();
     testReadAsTextSync();
     testReadAsLines();
     testReadAsLinesSync();
     testReadAsErrors();
 
@@ skipping 16 matching lines @@
       testWriteVariousLists();
       testDirectory();
       testDirectorySync();
     });
   }
 }
 
 main() {
   FileTest.testMain();
 }