Clean VM thread shutdown:

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 <stdlib.h>
 #include <string.h>
 #include <stdio.h>
@@ skipping 526 matching lines @@
 
   vm_options.AddArgument("--load_deferred_eagerly");
   // Workaround until issue 21620 is fixed.
   // (https://github.com/dart-lang/sdk/issues/21620)
   vm_options.AddArgument("--no-concurrent_sweep");
   Dart_SetVMFlags(vm_options.count(), vm_options.arguments());
 
   // Initialize the Dart VM.
   // Note: We don't expect isolates to be created from dart code during
   // snapshot generation.
-  if (!Dart_Initialize(NULL,
-                       NULL, NULL, NULL, NULL,
-                       DartUtils::OpenFile,
-                       DartUtils::ReadFile,
-                       DartUtils::WriteFile,
-                       DartUtils::CloseFile,
-                       DartUtils::EntropySource)) {
-    Log::PrintErr("VM initialization failed\n");
+  char* error = Dart_Initialize(NULL, NULL, NULL, NULL, NULL,
+      DartUtils::OpenFile,
+      DartUtils::ReadFile,
+      DartUtils::WriteFile,
+      DartUtils::CloseFile,
+      DartUtils::EntropySource);
+  if (error != NULL) {
+    Log::PrintErr("VM initialization failed: %s\n", error);
+    free(error);
     return 255;
   }
 
-  char* error;
   Dart_Isolate isolate = Dart_CreateIsolate(
       NULL, NULL, NULL, NULL, NULL, &error);
   if (isolate == NULL) {
     Log::PrintErr("Error: %s", error);
     free(error);
     exit(255);
   }
 
   Dart_Handle result;
   Dart_Handle library;
@@ skipping 62 matching lines @@
   }
   return 0;
 }
 
 }  // namespace bin
 }  // namespace dart
 
 int main(int argc, char** argv) {
   return dart::bin::main(argc, argv);
 }