Change file opening operation to set position at the end of the file when FileMode.APPEND is used.

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 "bin/file.h"
 
 #include <fcntl.h>
 #include <io.h>
 #include <stdio.h>
 #include <string.h>
@@ skipping 95 matching lines @@
   if ((mode & kWrite) != 0) {
     flags = (O_RDWR | O_CREAT | O_BINARY);
   }
   if ((mode & kTruncate) != 0) {
     flags = flags | O_TRUNC;
   }
   int fd = open(name, flags, 0666);
   if (fd < 0) {
     return NULL;
   }
+  if (((mode & kWrite) != 0) && ((mode & kTruncate) == 0)) {
+    int position = lseek(fd, 0, SEEK_END);
+    if (position < 0) {
+      return NULL;
+    }
+  }
   return new File(name, new FileHandle(fd));
 }
 
 
 bool File::Exists(const char* name) {
   struct stat st;
   if (stat(name, &st) == 0) {
     return ((st.st_mode & S_IFMT) == S_IFREG);
   } else {
     return false;
@@ skipping 38 matching lines @@
 
 
 const char* File::PathSeparator() {
   return "\\";
 }
 
 
 const char* File::StringEscapedPathSeparator() {
   return "\\\\";
 }