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

runtime/bin/gen_snapshot.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.
 
 // Generate a snapshot file after loading all the scripts specified on the
 // command line.
 
 #include <stdlib.h>
 #include <string.h>
 #include <stdio.h>
@@ skipping 87 matching lines @@
     i += 1;
   } else {
     *script_name = NULL;
   }
 
   return 0;
 }
 
 
 static void WriteSnapshotFile(const uint8_t* buffer, const intptr_t size) {
-  const bool kWritable = true;
-  File* file = File::Open(snapshot_filename, kWritable);
+  File* file = File::Open(snapshot_filename, File::kWriteTruncate);
   ASSERT(file != NULL);
   for (intptr_t i = 0; i < size; i++) {
     file->WriteByte(buffer[i]);
   }
   delete file;
 }
 
 
 static Dart_Handle CreateSnapshotLibraryTagHandler(Dart_LibraryTag tag,
                                                    Dart_Handle library,
@@ skipping 148 matching lines @@
     exit(255);
   }
   // Now write the snapshot out to specified file and exit.
   WriteSnapshotFile(buffer, size);
   Dart_ExitScope();
 
   // Shutdown the isolate.
   Dart_ShutdownIsolate();
   return 0;
 }