Change the behavior of open on files to not truncate by default

runtime/bin/file_macos.cc

 // Copyright (c) 2011, 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 <errno.h>
 #include <fcntl.h>
 #include <sys/stat.h>
 #include <unistd.h>
 #include <libgen.h>
 #include <limits.h>
@@ skipping 86 matching lines @@
   if (position < 0) {
     // The file is not capable of seeking. Return an error.
     return -1;
   }
   off_t result = lseek(handle_->fd(), 0, SEEK_END);
   lseek(handle_->fd(), position, SEEK_SET);
   return result;
 }
 
 
-File* File::Open(const char* name, bool writable) {
+File* File::Open(const char* name, FileOpenMode mode) {
   int flags = O_RDONLY;
-  if (writable) {
-    flags = (O_RDWR | O_CREAT | O_TRUNC);
+  if ((mode & kWrite) != 0) {
+    flags = (O_RDWR | O_CREAT);
+  }
+  if ((mode & kTruncate) != 0) {
+    flags = flags | O_TRUNC;
   }
   int fd = open(name, flags, 0666);
   if (fd < 0) {
     return NULL;
   }
   return new File(name, new FileHandle(fd));
 }
 
 
 bool File::Exists(const char* name) {
@@ skipping 46 matching lines @@
 
 
 const char* File::PathSeparator() {
   return "/";
 }
 
 
 const char* File::StringEscapedPathSeparator() {
   return "/";
 }