Signal an error for File.length on a directory

tests/standalone/io/file_test.dart

Index: tests/standalone/io/file_test.dart
diff --git a/tests/standalone/io/file_test.dart b/tests/standalone/io/file_test.dart
index 8a8d6811ec21c4dd8c9dd6b1d74ec5d004885473..0e632025ffd0005e13b1027a77c64d39d76ea890 100644
--- a/tests/standalone/io/file_test.dart
+++ b/tests/standalone/io/file_test.dart
@@ -642,6 +642,23 @@ class FileTest {
     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();
@@ -1428,6 +1445,7 @@ class FileTest {
 
     createTempDirectory(() {
       testLength();
+      testLengthSyncDirectory();
       testReadWrite();
       testReadWriteSync();
       testReadWriteNoArgsSync();