Signal an error for File.length on a directory

tests/standalone/io/file_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 file I/O.
 
 // OtherResources=empty_file
 // OtherResources=file_test.txt
 // OtherResources=fixed_length_file
 // OtherResources=read_as_text.dat
@@ skipping 624 matching lines @@
 
   static void testLengthSync() {
     String filename = getFilename("fixed_length_file");
     File file = new File(filename);
     RandomAccessFile openedFile = file.openSync();
     Expect.equals(42, file.lengthSync());
     Expect.equals(42, openedFile.lengthSync());
     openedFile.closeSync();
   }
 
+  static void testLengthSyncDirectory() {
+    Directory tmp = tempDirectory.createTempSync('file_length_test_');
+    String dirPath = '${tmp.path}/dir';
+    new Directory(dirPath).createSync();
+    try {
+      new File(dirPath).lengthSync();
+      Expect.fail('Expected operation to throw');
+    } catch (e) {
+      if (e is! FileSystemException) {
+        print(e);
+      }
+      Expect.isTrue(e is FileSystemException);
+    } finally {
+      tmp.deleteSync(recursive: true);
+    }
+  }
+
   // Test for file position functionality.
   static void testPosition() {
     asyncTestStarted();
     String filename = getFilename("fixed_length_file");
     RandomAccessFile input = (new File(filename)).openSync();
     input.position().then((position) {
       Expect.equals(0, position);
       List<int> buffer = new List<int>(100);
       input.readInto(buffer, 0, 12).then((bytes_read) {
         input.position().then((position) {
@@ skipping 766 matching lines @@
     testOpenDirectoryAsFileSync();
     testReadAsBytesSync();
     testReadAsBytesSyncEmptyFile();
     testReadAsTextSync();
     testReadAsTextSyncEmptyFile();
     testReadAsLinesSync();
     testLastModifiedSync();
 
     createTempDirectory(() {
       testLength();
+      testLengthSyncDirectory();
       testReadWrite();
       testReadWriteSync();
       testReadWriteNoArgsSync();
       testReadWriteStream();
       testReadEmptyFileSync();
       testReadEmptyFile();
       testReadWriteStreamLargeFile();
       testReadAsBytes();
       testReadAsBytesEmptyFile();
       testReadAsText();
@@ skipping 27 matching lines @@
       testLastModified();
       testDoubleAsyncOperation();
       asyncEnd();
     });
   }
 }
 
 main() {
   FileTest.testMain();
 }