Signal an error for File.length on a directory

runtime/bin/file_win.cc

 // Copyright (c) 2012, 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.
 
 #include "platform/globals.h"
 #if defined(TARGET_OS_WINDOWS)
 
 #include "bin/file.h"
 
 #include <fcntl.h>     // NOLINT
@@ skipping 426 matching lines @@
   }
   return false;
 }
 
 
 int64_t File::LengthFromPath(const char* name) {
   struct __stat64 st;
   Utf8ToWideScope system_name(name);
   int stat_status = _wstat64(system_name.wide(), &st);
   if (stat_status == 0) {
-    return st.st_size;
+    if ((st.st_mode & S_IFMT) == S_IFREG) {
+      return st.st_size;
+    } else {
+      // ERROR_DIRECTORY_NOT_SUPPORTED is not always in the message table.
+      SetLastError(ERROR_NOT_SUPPORTED);
+    }
   }
   return -1;
 }
 
 
 const char* File::LinkTarget(const char* pathname) {
   const wchar_t* name = StringUtilsWin::Utf8ToWide(pathname);
   HANDLE dir_handle = CreateFileW(
       name, GENERIC_READ,
       FILE_SHARE_READ | FILE_SHARE_WRITE | FILE_SHARE_DELETE, NULL,
@@ skipping 213 matching lines @@
     return kIdentical;
   } else {
     return kDifferent;
   }
 }
 
 }  // namespace bin
 }  // namespace dart
 
 #endif  // defined(TARGET_OS_WINDOWS)