Support length operation without opening file.

runtime/bin/builtin_natives.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 "bin/builtin.h"
 
 #include <stdlib.h>
 #include <string.h>
 
 #include "bin/dartutils.h"
@@ skipping 18 matching lines @@
   V(File_Close, 1)                                                             \
   V(File_ReadByte, 1)                                                          \
   V(File_WriteByte, 2)                                                         \
   V(File_WriteString, 2)                                                       \
   V(File_ReadList, 4)                                                          \
   V(File_WriteList, 4)                                                         \
   V(File_Position, 1)                                                          \
   V(File_SetPosition, 2)                                                       \
   V(File_Truncate, 2)                                                          \
   V(File_Length, 1)                                                            \
+  V(File_LengthFromName, 1)                                                    \
   V(File_Flush, 1)                                                             \
   V(File_Create, 1)                                                            \
   V(File_Delete, 1)                                                            \
   V(File_Directory, 1)                                                         \
   V(File_FullPath, 1)                                                          \
   V(File_OpenStdio, 1)                                                         \
   V(File_GetStdioHandleType, 1)                                                \
   V(File_NewServicePort, 0)                                                    \
   V(Logger_PrintString, 1)                                                     \
   V(Platform_NumberOfProcessors, 0)                                            \
@@ skipping 88 matching lines @@
   } else {
     ASSERT(id == Builtin::kIOLibrary);
     url = Dart_NewString(DartUtils::kIOLibURL);
   }
   Dart_Handle builtin_lib = Dart_LookupLibrary(url);
   DART_CHECK_VALID(builtin_lib);
   // Setup the native resolver for built in library functions.
   Dart_Handle result = Dart_SetNativeResolver(builtin_lib, NativeLookup);
   DART_CHECK_VALID(result);
 }