Exit gen_snapshot and log an error if an output file path is invalid

runtime/bin/gen_snapshot.cc

 // Copyright (c) 2013, 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 <stdio.h>
 #include <stdlib.h>
 #include <string.h>
@@ skipping 364 matching lines @@
   }
 
   return 0;
 }
 
 
 static void WriteSnapshotFile(const char* filename,
                               const uint8_t* buffer,
                               const intptr_t size) {
   File* file = File::Open(filename, File::kWriteTruncate);
-  ASSERT(file != NULL);
+  if (file == NULL) {
+    Log::PrintErr("Error: Unable to write snapshot file: %s\n\n", filename);
+    Dart_ExitScope();
+    Dart_ShutdownIsolate();
+    exit(kErrorExitCode);
+  }
   if (!file->WriteFully(buffer, size)) {
     Log::PrintErr("Error: Failed to write snapshot file.\n\n");
   }
   file->Release();
 }
 
 
 class UriResolverIsolateScope {
  public:
   UriResolverIsolateScope() {
@@ skipping 969 matching lines @@
   EventHandler::Stop();
   return 0;
 }
 
 }  // namespace bin
 }  // namespace dart
 
 int main(int argc, char** argv) {
   return dart::bin::main(argc, argv);
 }