Signal an error for File.length on a directory

runtime/bin/file_fuchsia.cc

 // Copyright (c) 2016, 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_FUCHSIA)
 
 #include "bin/file.h"
 
 #include <errno.h>      // NOLINT
@@ skipping 329 matching lines @@
   } else {
     errno = ENOENT;
   }
   return false;
 }
 
 
 int64_t File::LengthFromPath(const char* name) {
   struct stat st;
   if (NO_RETRY_EXPECTED(stat(name, &st)) == 0) {
+    // Signal an error if it's a directory.
+    if (S_ISDIR(st.st_mode)) {
+      errno = EISDIR;
+      return -1;
+    }
+    // Otherwise assume the caller knows what it's doing.
     return st.st_size;
   }
   return -1;
 }
 
 
 void File::Stat(const char* name, int64_t* data) {
   struct stat st;
   if (NO_RETRY_EXPECTED(stat(name, &st)) == 0) {
     if (S_ISREG(st.st_mode)) {
@@ skipping 141 matching lines @@
   return ((file_1_info.st_ino == file_2_info.st_ino) &&
           (file_1_info.st_dev == file_2_info.st_dev))
              ? File::kIdentical
              : File::kDifferent;
 }
 
 }  // namespace bin
 }  // namespace dart
 
 #endif  // defined(TARGET_OS_FUCHSIA)