Added API Dart_PostCMessage for posting a Dart_CMessage structure

runtime/vm/snapshot.h

Index: runtime/vm/snapshot.h
diff --git a/runtime/vm/snapshot.h b/runtime/vm/snapshot.h
index 045d36f19e36101798b6898f0acb50746ab6da17..00a79540df62fbe158d88c5254c757f32beb0f3d 100644
--- a/runtime/vm/snapshot.h
+++ b/runtime/vm/snapshot.h
@@ -535,18 +535,26 @@ class BaseWriter {
 
 class MessageWriter : public BaseWriter {
  public:
-  MessageWriter(uint8_t** buffer, ReAlloc alloc) : BaseWriter(buffer, alloc) {
+  MessageWriter(uint8_t** buffer, ReAlloc alloc) : BaseWriter(buffer, alloc),
+                                                   object_id_(0) {
   }
   ~MessageWriter() { }
 
   // Writes a message of integers.
   void WriteMessage(intptr_t field_count, intptr_t *data);
 
+  void WriteCMessage(Dart_CObject* object);
+
   void FinalizeBuffer() {
     BaseWriter::FinalizeBuffer(Snapshot::kMessage);
   }
 
  private:
+  void WriteSmi(int32_t value);
+  void WriteCObject(Dart_CObject* object);
+
+  intptr_t object_id_;
+
   DISALLOW_COPY_AND_ASSIGN(MessageWriter);
 };