Fail cleanly if we cannot open a file to write a snapshot.

runtime/bin/main.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 <stdlib.h>
 #include <string.h>
 #include <stdio.h>
 
 #include "include/dart_api.h"
 #include "include/dart_tools_api.h"
@@ skipping 1042 matching lines @@
   if ((snapshot_directory != NULL) && (strlen(snapshot_directory) > 0)) {
     intptr_t len = snprintf(NULL, 0, "%s/%s", snapshot_directory, filename);
     concat = new char[len + 1];
     snprintf(concat, len + 1, "%s/%s", snapshot_directory, filename);
     qualified_filename = concat;
   } else {
     qualified_filename = filename;
   }
 
   File* file = File::Open(qualified_filename, File::kWriteTruncate);
-  ASSERT(file != NULL);
+  if (file == NULL) {
+    ErrorExit(kErrorExitCode,
+              "Unable to open file %s for writing snapshot\n",
+              qualified_filename);
+  }
 
   if (write_magic_number) {
     // Write the magic number to indicate file is a script snapshot.
     DartUtils::WriteMagicNumber(file);
   }
 
   if (!file->WriteFully(buffer, size)) {
     ErrorExit(kErrorExitCode,
-              "Unable to open file %s for writing snapshot\n",
+              "Unable to write file %s for writing snapshot\n",
               qualified_filename);
   }
   file->Release();
   if (concat != NULL) {
     delete concat;
   }
 }
 
 
 static void ReadSnapshotFile(const char* snapshot_directory,
@@ skipping 662 matching lines @@
   Platform::Exit(Process::GlobalExitCode());
 }
 
 }  // namespace bin
 }  // namespace dart
 
 int main(int argc, char** argv) {
   dart::bin::main(argc, argv);
   UNREACHABLE();
 }