Avoid trusting the length encoded in the Snapshot if there is an

runtime/vm/dart_api_message.h

Index: runtime/vm/dart_api_message.h
===================================================================
--- runtime/vm/dart_api_message.h	(revision 10993)
+++ runtime/vm/dart_api_message.h	(working copy)
@@ -27,7 +27,7 @@
 // Reads a message snapshot into a C structure.
 class ApiMessageReader : public BaseReader {
  public:
-  ApiMessageReader(const uint8_t* buffer, intptr_t length, ReAlloc alloc);
+  ApiMessageReader(const Snapshot* snapshot, ReAlloc alloc);
   ~ApiMessageReader() { }
 
   Dart_CObject* ReadMessage();
@@ -122,12 +122,12 @@
   }
 
   // Writes a message of integers.
-  void WriteMessage(intptr_t field_count, intptr_t *data);
+  intptr_t WriteMessage(intptr_t field_count, intptr_t *data);
 
-  void WriteCMessage(Dart_CObject* object);
+  intptr_t WriteCMessage(Dart_CObject* object);
 
-  void FinalizeBuffer() {
-    BaseWriter::FinalizeBuffer(Snapshot::kMessage);
+  intptr_t FinalizeBuffer() {
+    return BaseWriter::FinalizeBuffer(Snapshot::kMessage);
   }
 
  private: