Fix errors in checked mode

tests/standalone/io/file_test.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:async';
 import 'dart:io';
 import 'dart:isolate';
 
@@ skipping 207 matching lines @@
           Expect.equals(103, buffer[9]);  // represents 'g' in the file.
           file.close().then((ignore) => port.close());
         });
       });
     });
   }
 
   static void testReadSync() {
     // Read a file and check part of it's contents.
     String filename = getFilename("bin/file_test.cc");
-    RandomAccessFile file = (new File(filename)).openSync();
+    RandomAccessFile raf = (new File(filename)).openSync();
     List<int> buffer = new List<int>(42);
     int bytes_read = 0;
-    bytes_read = file.readListSync(buffer, 0, 12);
+    bytes_read = raf.readListSync(buffer, 0, 12);
     Expect.equals(12, bytes_read);
-    bytes_read = file.readListSync(buffer, 12, 30);
+    bytes_read = raf.readListSync(buffer, 12, 30);
     Expect.equals(30, bytes_read);
     Expect.equals(47, buffer[0]);  // represents '/' in the file.
     Expect.equals(47, buffer[1]);  // represents '/' in the file.
     Expect.equals(32, buffer[2]);  // represents ' ' in the file.
     Expect.equals(67, buffer[3]);  // represents 'C' in the file.
     Expect.equals(111, buffer[4]);  // represents 'o' in the file.
     Expect.equals(112, buffer[5]);  // represents 'p' in the file.
     Expect.equals(121, buffer[6]);  // represents 'y' in the file.
     Expect.equals(114, buffer[7]);  // represents 'r' in the file.
     Expect.equals(105, buffer[8]);  // represents 'i' in the file.
     Expect.equals(103, buffer[9]);  // represents 'g' in the file.
     Expect.equals(104, buffer[10]);  // represents 'h' in the file.
     Expect.equals(116, buffer[11]);  // represents 't' in the file.
 
     filename = getFilename("tests/vm/data/fixed_length_file");
-    file = new File(filename);
+    File file = new File(filename);
     int len = file.lengthSync();
     Expect.equals(0, file.openSync().readSync(0).length);
     Expect.equals(1, file.openSync().readSync(1).length);
     Expect.equals(len - 1, file.openSync().readSync(len - 1).length);
     Expect.equals(len, file.openSync().readSync(len).length);
     Expect.equals(len, file.openSync().readSync(len + 1).length);
     Expect.equals(len, file.openSync().readSync(len * 2).length);
     Expect.equals(len, file.openSync().readSync(len * 10).length);
   }
 
@@ skipping 972 matching lines @@
       testDirectorySync();
       testWriteStringUtf8();
       testWriteStringUtf8Sync();
     });
   }
 }
 
 main() {
   FileTest.testMain();
 }