Fix some potential data races based on output from tsan.

runtime/bin/gen_snapshot.cc

Index: runtime/bin/gen_snapshot.cc
diff --git a/runtime/bin/gen_snapshot.cc b/runtime/bin/gen_snapshot.cc
index ea9f3d7bf0a1b4e4fe823668825ca9a898367c6b..45087b4c502233d3ccac6f0c491e74078ce5a663 100644
--- a/runtime/bin/gen_snapshot.cc
+++ b/runtime/bin/gen_snapshot.cc
@@ -1142,6 +1142,11 @@ int main(int argc, char** argv) {
     SetupForGenericSnapshotCreation();
     CreateAndWriteSnapshot();
   }
+  error = Dart_Cleanup();
+  if (error != NULL) {
+    Log::PrintErr("VM cleanup failed: %s\n", error);
+    free(error);
+  }
   EventHandler::Stop();
   return 0;
 }