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

runtime/bin/file_macos.cc

Index: runtime/bin/file_macos.cc
===================================================================
--- runtime/bin/file_macos.cc	(revision 3943)
+++ runtime/bin/file_macos.cc	(working copy)
@@ -117,6 +117,10 @@
   if (fd < 0) {
     return NULL;
   }
+  if (((mode & kWrite) != 0) && ((mode & kTruncate) == 0)) {
+    int position = TEMP_FAILURE_RETRY(lseek(fd, 0, SEEK_END));
+    ASSERT(position >= 0);
+  }
   return new File(name, new FileHandle(fd));
 }