Reapply "Use a single file for app snapshots."

runtime/vm/clustered_snapshot.cc

Index: runtime/vm/clustered_snapshot.cc
diff --git a/runtime/vm/clustered_snapshot.cc b/runtime/vm/clustered_snapshot.cc
index d9e65ad479666f2d11549b0fe08dc149498abdc5..1799db88ae9e126563c3aec0fa16fb193cf7caf1 100644
--- a/runtime/vm/clustered_snapshot.cc
+++ b/runtime/vm/clustered_snapshot.cc
@@ -5287,7 +5287,10 @@ void FullSnapshotWriter::WriteFullSnapshot() {
   WriteIsolateFullSnapshot(num_base_objects);
 
   if (Snapshot::IncludesCode(kind_)) {
-    instructions_writer_->Write();
+    instructions_writer_->Write(*vm_isolate_snapshot_buffer_,
+                                vm_isolate_snapshot_size_,
+                                *isolate_snapshot_buffer_,
+                                isolate_snapshot_size_);
 
     OS::Print("VMIsolate(CodeSize): %" Pd "\n", VmIsolateSnapshotSize());
     OS::Print("Isolate(CodeSize): %" Pd "\n", IsolateSnapshotSize());