dart - add Packable interface (#6846)

dart/lib/flat_buffers.dart

@@ -77,6 +77,12 @@ class BufferContext {
   int _getUint8(int offset) => _buffer.getUint8(offset);
 }
 
+/// Interface implemented by the "object-api" classes (ending with "T").
+abstract class Packable {
+  /// Serialize the object using the given builder, returning the offset.
+  int pack(Builder fbBuilder);
+}
+
 /// Class implemented by typed builders generated by flatc.
 abstract class ObjectBuilder {
   int? _firstOffset;