Add support for lists and backward references when decoding a message to a Dart_CObject object

runtime/include/dart_api.h

 // Copyright (c) 2012, 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.
 
 #ifndef INCLUDE_DART_API_H_
 #define INCLUDE_DART_API_H_
 
 /** \mainpage Dart Embedding API Reference
  *
  * Dart is a class-based programming language for creating structured
@@ skipping 1398 matching lines @@
 // --- Message encoding/decoding ----
 
 /**
  * A Dart_CObject is used for representing Dart objects as native C
  * data outside the Dart heap. These objects are totally detached from
  * the Dart heap. Only a subset of the Dart objects have a
  * representation as a Dart_CObject.
  */
 struct Dart_CObject {
   enum Type {
-    kNull,
+    kNull = 0,
     kBool,
     kInt32,
     kDouble,
     kString,
-    kArray
+    kArray,
+    kNumberOfTypes
   };
   Type type;
   union {
     bool as_bool;
     int32_t as_int32;
     double as_double;
     char* as_string;
     struct {
       int length;
       Dart_CObject** values;
     } as_array;
   } value;
 };
 
 /**
  * A Dart_CMessage is used for encoding and decoding messages from native code.
  */
 struct Dart_CMessage {
   Dart_CObject** message;
 };
 
 #endif  // INCLUDE_DART_API_H_