Dart, Endianness in native structs

Question

I have the following Structs in Dart:

final class Header extends Struct {
      @Uint16()
      external int version;
      @Uint16()
      external int type;
      @Uint32()
      external int size;
}

final class Payload extends Struct {
      @Uint32()
      external int x;

      @Uint32()
      external int y;
}

It's supposed to represent a Header + Payload for a network packet. I am aware that I can use ByteData to write the fields of each Struct manually to, for example, a Uint8List. However, I'd like to use the defined structure to create/parse packets. Currently, the idea is allocating memory for a complete packet, and then casting it to the Struct type.

1. How would I set the fields of the Struct so that they would be stored in NBO/Big Endian in memory?
2. How would I "map" multiple Structs to an area of allocated memory with offsets, similar to what would be done in C like this:

typedef struct __attribute__((packed)) {
        uint32_t foo;
} s1;

typedef struct __attribute__((packed)) {
        uint32_t bar;
} s2;

void *buf = malloc(sizeof s1 + sizeof s2);

s1 *a = buf;
s2 *b = buf + sizeof s1;

/* ... */

I'd like to get around writing my packet building/parsing in C and having to interface between dart in C as a consequence.