Add comments to the synchronous file methods stating that they throw

sdk/lib/io/file_impl.dart

Index: sdk/lib/io/file_impl.dart
diff --git a/sdk/lib/io/file_impl.dart b/sdk/lib/io/file_impl.dart
index 8ab05f6c5730b2b22159d85da821d1e590107d1a..6da4d9f4f2c0ddf5b412a19a544c0d58e2ef0bee 100644
--- a/sdk/lib/io/file_impl.dart
+++ b/sdk/lib/io/file_impl.dart
@@ -698,11 +698,17 @@ class _RandomAccessFile extends _FileBase implements RandomAccessFile {
   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) {