[vm, isolate] Refactor isolate message snapshotting to centralize construction of the Message.

Remove unused special case in ApiMessageWriter for lists of int.

This is in preparation for ensuring we always free any external data that ends up in an isolate message.

Bug: https://github.com/dart-lang/sdk/issues/31959
Change-Id: I999656fc11d2aee9aebe70852be5bb075f234b4d
Reviewed-on: https://dart-review.googlesource.com/41020
Reviewed-by: Zach Anderson <zra@google.com>
Commit-Queue: Ryan Macnak <rmacnak@google.com>

runtime/vm/dart_api_message.h

@@ -47,7 +47,6 @@ class ApiMessageReader : public BaseReader {
   // The ApiMessageReader object must be enclosed by an ApiNativeScope.
   // Allocation of all C Heap objects is done in the zone associated with
   // the enclosing ApiNativeScope.
-  ApiMessageReader(const uint8_t* buffer, intptr_t length);
   explicit ApiMessageReader(Message* message);
   ~ApiMessageReader() {}
 
@@ -155,21 +154,13 @@ class ApiMessageReader : public BaseReader {
 class ApiMessageWriter : public BaseWriter {
  public:
   static const intptr_t kInitialSize = 512;
-  ApiMessageWriter(uint8_t** buffer, ReAlloc alloc)
-      : BaseWriter(buffer, alloc, NULL, kInitialSize),
-        object_id_(0),
-        forward_list_(NULL),
-        forward_list_length_(0),
-        forward_id_(0) {
-    ASSERT(kDartCObjectTypeMask >= Dart_CObject_kNumberOfTypes - 1);
-  }
-  ~ApiMessageWriter() { ::free(forward_list_); }
-
-  // Writes a message of integers.
-  void WriteMessage(intptr_t field_count, intptr_t* data);
+  ApiMessageWriter();
+  ~ApiMessageWriter();
 
   // Writes a message with a single object.
-  bool WriteCMessage(Dart_CObject* object);
+  Message* WriteCMessage(Dart_CObject* object,
+                         Dart_Port dest_port,
+                         Message::Priority priority);
 
  private:
   static const intptr_t kDartCObjectTypeBits = 4;