Add comments to the synchronous file methods stating that they throw

sdk/lib/io/file_impl.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.
 
 part of dart.io;
 
 // Read the file in blocks of size 64k.
 const int _BLOCK_SIZE = 64 * 1024;
 
 
@@ skipping 680 matching lines @@
         throw _exceptionFromResponse(response,
                                      "read failed for file '$_path'");
       }
       return response[1];
     });
   }
 
   external static _read(int id, int bytes);
 
   List<int> readSync(int bytes) {
+    _checkNotClosed();
     if (bytes is !int) {
       throw new FileIOException(
           "Invalid arguments to readSync for file '$_path'");
     }
-    return _read(_id, bytes);
+    var result = _read(_id, bytes);
+    if (result is OSError) {
+      throw new FileIOException("readSync failed for file '$_path'",
+                                result);
+    }
+    return result;
   }
 
   Future<int> readList(List<int> buffer, int offset, int bytes) {
     _ensureFileService();
     Completer<int> completer = new Completer<int>();
     if (buffer is !List || offset is !int || bytes is !int) {
       // Complete asynchronously so the user has a chance to setup
       // handlers without getting exceptions when registering the
       // then handler.
       Timer.run(() {
@@ skipping 328 matching lines @@
           new FileIOException("File closed '$_path'"));
     });
     return completer.future;
   }
 
   final String _path;
   int _id;
 
   SendPort _fileService;
 }