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

runtime/bin/file_macos.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 <errno.h>
 #include <fcntl.h>
 #include <sys/stat.h>
 #include <unistd.h>
@@ skipping 99 matching lines @@
   if ((mode & kWrite) != 0) {
     flags = (O_RDWR | O_CREAT);
   }
   if ((mode & kTruncate) != 0) {
     flags = flags | O_TRUNC;
   }
   int fd = TEMP_FAILURE_RETRY(open(name, flags, 0666));
   if (fd < 0) {
     return NULL;
   }
+  if (((mode & kWrite) != 0) && ((mode & kTruncate) == 0)) {
+    int position = TEMP_FAILURE_RETRY(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 (TEMP_FAILURE_RETRY(stat(name, &st)) == 0) {
     return S_ISREG(st.st_mode);  // Deal with symlinks?
   } else {
     return false;
@@ skipping 43 matching lines @@
 
 
 const char* File::PathSeparator() {
   return "/";
 }
 
 
 const char* File::StringEscapedPathSeparator() {
   return "/";
 }