パターン1
var httpsUri = Uri(
scheme: 'https',
host: 'example.com',
path: '/page/',
);
print(httpsUri);
httpsUri = httpsUri.resolve("../aaa.txt");
print(httpsUri);
もしくは
var httpsUri = Uri.parse("https://example.com/page/");
print(httpsUri);
httpsUri = httpsUri.resolve("../aaa.txt");
print(httpsUri);
結果
https://example.com/page/
https://example.com/aaa.txt
パターン2
var httpsUri = Uri.parse("https://example.com/page/bbb.txt");
print(httpsUri);
httpsUri = httpsUri.resolve("../aaa.txt");
print(httpsUri);
結果
https://example.com/page/bbb.txt
https://example.com/aaa.txt