Move MessageWriter from snapshot files to dart_api_message files

runtime/vm/dart_api_message.h

Index: runtime/vm/dart_api_message.h
diff --git a/runtime/vm/dart_api_message.h b/runtime/vm/dart_api_message.h
index d8a4339e7f5ea369b9c108cab584c33b5714e0c1..76a97884f4abab28d1ba37f437b10805a5caecbb 100644
--- a/runtime/vm/dart_api_message.h
+++ b/runtime/vm/dart_api_message.h
@@ -82,6 +82,48 @@ class ApiMessageReader : public BaseReader {
   Dart_CObject dynamic_type_marker;
 };
 
+
+class ApiMessageWriter : public BaseWriter {
+ public:
+  ApiMessageWriter(uint8_t** buffer, ReAlloc alloc)
+      : BaseWriter(buffer, alloc), object_id_(0) {
+    ASSERT(kDartCObjectTypeMask >= Dart_CObject::kNumberOfTypes - 1);
+  }
+  ~ApiMessageWriter() { }
+
+  // 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:
+  static const intptr_t kDartCObjectTypeBits = 4;
+  static const intptr_t kDartCObjectTypeMask = (1 << kDartCObjectTypeBits) - 1;
+  static const intptr_t kDartCObjectMarkMask = ~kDartCObjectTypeMask;
+  static const intptr_t kDartCObjectMarkOffset = 1;
+
+  void MarkCObject(Dart_CObject* object, intptr_t object_id);
+  void UnmarkCObject(Dart_CObject* object);
+  bool IsCObjectMarked(Dart_CObject* object);
+  intptr_t GetMarkedCObjectMark(Dart_CObject* object);
+  void UnmarkAllCObjects(Dart_CObject* object);
+
+  void WriteSmi(int64_t value);
+  void WriteMint(Dart_CObject* object, int64_t value);
+  void WriteInt32(Dart_CObject* object);
+  void WriteInt64(Dart_CObject* object);
+  void WriteInlinedHeader(Dart_CObject* object);
+  void WriteCObject(Dart_CObject* object);
+
+  intptr_t object_id_;
+
+  DISALLOW_COPY_AND_ASSIGN(ApiMessageWriter);
+};
+
 }  // namespace dart
 
 #endif  // VM_DART_API_MESSAGE_H_