Support receiving GrowableObjectArray on native ports

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 2314315371adb6ae37274332725d8a6ee05a9ec7..5b131bd7a845788147ccac55a2ffdb69632cd175 100644
--- a/runtime/vm/dart_api_message.h
+++ b/runtime/vm/dart_api_message.h
@@ -27,6 +27,11 @@ struct Dart_CObject_Internal : public Dart_CObject {
 // Reads a message snapshot into a C structure.
 class ApiMessageReader : public BaseReader {
  public:
+  // The allocator passed is used to allocate memory for the C structure used
+  // to represent the message snapshot. This allocator must keep track of the
+  // memory allocated as there is no way to run through the resulting C
+  // structure and free the individual pieces. Using a zone based allocator is
+  // recommended.
   ApiMessageReader(const uint8_t* buffer, intptr_t length, ReAlloc alloc);
   ~ApiMessageReader() { }