Allocate a Dart_CMessage structure when decoding a message into C structures

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
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 // TODO(turnidge): Rename to Dart_LibrarySetNativeResolver?
 
 // --- Profiling support ----
 
 // External pprof support for gathering and dumping symbolic
 // information that can be used for better profile reports for
 // dynamically generated code.
 DART_EXPORT void Dart_InitPprofSupport();
 DART_EXPORT void Dart_GetPprofSymbolInfo(void** buffer, int* buffer_size);
 
-// --- Message encoding/decoding ----
+// --- Message sending/receiving from native code ----
 
 /**
  * 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 = 0,
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     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.
+ * A Dart_CMessage is used for receiving and sending messages from
+ * native code not running in an isolate. A message contains an object
+ * graph represented as Dart_CObject structures rooted as the provided
+ * root.
+ *
+ * For information on the lifetime of this data, when provided in
+ * callbacks, see the documentation for the individual callbacks.
  */
 struct Dart_CMessage {
-  Dart_CObject** message;
+  Dart_CObject* root;
 };
 
+
 #endif  // INCLUDE_DART_API_H_