Unify specification of file open, close, and write callbacks.

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 ac50f47cd82336cc93b62317c6bd93835d78ed6d..830d8bcb6745e24a311184b7b0edbf2ee9aa9456 100644
--- a/runtime/bin/gen_snapshot.cc
+++ b/runtime/bin/gen_snapshot.cc
@@ -301,7 +301,8 @@ int main(int argc, char** argv) {
   // 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)) {
+  if (!Dart_Initialize(NULL, NULL, NULL, NULL,
+                       NULL, NULL, NULL)) {
     Log::PrintErr("VM initialization failed\n");
     return 255;
   }