[vm] Add Dart_CreateVMAOTSnapshotAsAssembly.

runtime/vm/clustered_snapshot.cc

 // Copyright (c) 2016, 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.
 
 #include "vm/clustered_snapshot.h"
 
 #include "platform/assert.h"
 #include "vm/bootstrap.h"
 #include "vm/class_finalizer.h"
 #include "vm/dart.h"
@@ skipping 5436 matching lines @@
 
 void FullSnapshotWriter::WriteFullSnapshot() {
   intptr_t num_base_objects;
   if (vm_snapshot_data_buffer() != NULL) {
     num_base_objects = WriteVMSnapshot();
     ASSERT(num_base_objects != 0);
   } else {
     num_base_objects = 0;
   }
 
-  WriteIsolateSnapshot(num_base_objects);
+  if (isolate_snapshot_data_buffer() != NULL) {
+    WriteIsolateSnapshot(num_base_objects);
+  }
 
   if (FLAG_print_snapshot_sizes) {
     OS::Print("VMIsolate(CodeSize): %" Pd "\n", clustered_vm_size_);
     OS::Print("Isolate(CodeSize): %" Pd "\n", clustered_isolate_size_);
     OS::Print("ReadOnlyData(CodeSize): %" Pd "\n", mapped_data_size_);
     OS::Print("Instructions(CodeSize): %" Pd "\n", mapped_instructions_size_);
     OS::Print("Total(CodeSize): %" Pd "\n",
               clustered_vm_size_ + clustered_isolate_size_ + mapped_data_size_ +
                   mapped_instructions_size_);
   }
@@ skipping 61 matching lines @@
     thread_->isolate()->SetupImagePage(data_buffer_,
                                        /* is_executable */ false);
   }
 
   deserializer.ReadIsolateSnapshot(thread_->isolate()->object_store());
 
   return ApiError::null();
 }
 
 }  // namespace dart