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+/// A throwaway HTTP server on a random loopback port.
+///
+/// The suite points a real `Client` at a real socket: no test touches the
+/// network, and no test stubs the HTTP layer. The code path exercised is the
+/// one that runs in production, request construction and error body reading
+/// included.
+///
+/// It doubles as the test that `baseUrl` works — if it did not, none of this
+/// would run.
+library;
+
+import 'dart:async';
+import 'dart:convert';
+import 'dart:io';
+
+/// What the fake server should answer for one request.
+class Reply {
+  /// Creates an answer with [status], [body] and any extra [headers].
+  const Reply(this.status, this.body,
+      {this.headers = const <String, String>{}});
+
+  /// HTTP status to answer with.
+  final int status;
+
+  /// Body to write, encoded as UTF-8.
+  final String body;
+
+  /// Extra response headers, such as `Retry-After`.
+  final Map<String, String> headers;
+}
+
+/// Per-test programmable routes, plus a record of what was asked.
+class Router {
+  final Map<String, List<Reply>> _routes = <String, List<Reply>>{};
+
+  /// Every path requested so far, in order, query string included.
+  final List<String> calls = <String>[];
+
+  /// The `User-Agent` of every request so far, in order.
+  final List<String> userAgents = <String>[];
+
+  /// Answers any path with no registered route.
+  Reply Function(String path)? fallback;
+
+  /// Registers the single answer for [path].
+  void respond(String path, Reply reply) {
+    _routes[path] = <Reply>[reply];
+  }
+
+  /// Registers a sequence of answers for [path], one per request.
+  ///
+  /// The last entry repeats once the sequence runs out, which is how retrying
+  /// is tested.
+  void respondInSequence(String path, List<Reply> replies) {
+    _routes[path] = List<Reply>.of(replies);
+  }
+
+  /// Records the call and returns the answer registered for [path].
+  Reply next(String path) {
+    calls.add(path);
+    final replies = _routes[path];
+    if (replies == null) {
+      final handler = fallback;
+      if (handler != null) {
+        return handler(path);
+      }
+      return Reply(
+          404, jsonEncode(<String, String>{'erro': 'cep_nao_encontrado'}));
+    }
+    return replies.length > 1 ? replies.removeAt(0) : replies.first;
+  }
+
+  /// How many requests so far began with [prefix].
+  int count(String prefix) =>
+      calls.where((String call) => call.startsWith(prefix)).length;
+}
+
+/// A running fake API, with its router.
+class FakeServer {
+  FakeServer._(this._server, this.routes);
+
+  final HttpServer _server;
+
+  /// The routes this server answers from.
+  final Router routes;
+
+  /// Starts a fake API on `127.0.0.1` and a port the kernel picks.
+  static Future<FakeServer> start() async {
+    final server = await HttpServer.bind(InternetAddress.loopbackIPv4, 0);
+    final fake = FakeServer._(server, Router());
+    unawaited(fake._serve());
+    return fake;
+  }
+
+  /// Where to point a client's `baseUrl`.
+  String get url => 'http://127.0.0.1:${_server.port}';
+
+  /// Stops the server and drops any open connection.
+  Future<void> stop() => _server.close(force: true);
+
+  Future<void> _serve() async {
+    await for (final HttpRequest request in _server) {
+      await request.drain<void>();
+      routes.userAgents.add(
+        request.headers.value(HttpHeaders.userAgentHeader) ?? '',
+      );
+
+      final uri = request.uri;
+      final path = uri.query.isEmpty ? uri.path : '${uri.path}?${uri.query}';
+      final reply = routes.next(path);
+
+      final response = request.response
+        ..statusCode = reply.status
+        ..headers.contentType = ContentType(
+          'application',
+          'json',
+          charset: 'utf-8',
+        );
+      for (final MapEntry<String, String> header in reply.headers.entries) {
+        response.headers.set(header.key, header.value);
+      }
+      response.add(utf8.encode(reply.body));
+      await response.close();
+    }
+  }
+}