Add comments to the synchronous file methods stating that they throw

tests/standalone/io/file_error_test.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.
 //
 // Dart test program for testing error handling in file I/O.
 
 import "dart:io";
 import "dart:isolate";
 
 Directory tempDir() {
@@ skipping 435 matching lines @@
 testRepeatedlyCloseFileSync() {
   createTestFile((file, port) {
     var openedFile = file.openSync();
     openedFile.closeSync();
     Expect.throws(openedFile.closeSync,
                   (e) => e is FileIOException);
     port.send(null);
   });
 }
 
+testReadSyncBigInt() {
+  createTestFile((file, port) {
+    var bigint = 100000000000000000000000000000000000000000;
+    var openedFile = file.openSync();
+    Expect.throws(() => openedFile.readSync(bigint),
+                  (e) => e is FileIOException);
+    port.send(null);
+  });
+}
+
+testReadSyncClosedFile() {
+  createTestFile((file, port) {
+    var openedFile = file.openSync();
+    openedFile.closeSync();
+    Expect.throws(() => openedFile.readSync(1),
+                  (e) => e is FileIOException);
+    port.send(null);
+  });
+}
+
 main() {
   testOpenNonExistent();
   testDeleteNonExistent();
   testLengthNonExistent();
   testCreateInNonExistentDirectory();
   testFullPathOnNonExistentDirectory();
   testDirectoryInNonExistentDirectory();
   testReadAsBytesNonExistent();
   testReadAsTextNonExistent();
   testReadAsLinesNonExistent();
   testWriteByteToReadOnlyFile();
   testWriteListToReadOnlyFile();
   testTruncateReadOnlyFile();
   testOperateOnClosedFile();
   testRepeatedlyCloseFile();
   testRepeatedlyCloseFileSync();
+  testReadSyncBigInt();
+  testReadSyncClosedFile();
 }