Amplify Flutter의 API입니다.Post/put(REST)에서 일본어 난자

try {
    RestOptions options = RestOptions(
        path: '/todo',
        body: Uint8List.fromList('{\'name\':\'ほげ\'}'.codeUnits) // <------- 変更箇所 Uint8List.fromList で文字列変換すると、日本語が文字化けします。
    );
    RestOperation restOperation = Amplify.API.put(
        restOptions: options
    );
    RestResponse response = await restOperation.response;
    print('PUT call succeeded');
    print(new String.fromCharCodes(response.data));
} on ApiException catch (e) {
    print('PUT call failed: $e');
}

import 'dart:convert';

...(中略)...

try {
    final encoder = const Utf8Encoder();  // <------- 追加
    RestOptions options = RestOptions(
        path: '/todo',
        body: encoder.convert(('{\'name\':\'ほげ\'}') // <------- 変更箇所 Utf8Encoder().convert でbyte配列に変更しましょう。
    );
    RestOperation restOperation = Amplify.API.put(
        restOptions: options
    );
    RestResponse response = await restOperation.response;
    print('PUT call succeeded');
    print(new String.fromCharCodes(response.data));
} on ApiException catch (e) {
    print('PUT call failed: $e');
}

참고 자료