gen_snapshot: When copying an error across isolates, remember to turn it back into an error.

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 430 matching lines @@
       result_string = strdup(Dart_GetError(result));
     }
   }
   // Switch back to the isolate from which we generate the snapshot and
   // create the source string for the specified uri.
   Dart_Handle result;
   if (!failed) {
     result = Dart_NewStringFromUTF8(payload, payload_length);
     free(payload);
   } else {
-    result = DartUtils::NewString(result_string);
+    result = Dart_NewApiError(result_string);
     free(result_string);
   }
   return result;
 }
 
 
 static Dart_Handle ResolveUriInWorkingDirectory(const char* script_uri) {
   bool failed = false;
   char* result_string = NULL;
 
@@ skipping 921 matching lines @@
   EventHandler::Stop();
   return 0;
 }
 
 }  // namespace bin
 }  // namespace dart
 
 int main(int argc, char** argv) {
   return dart::bin::main(argc, argv);
 }