fix diagnostics in the FFI sample

Change-Id: I9697c33be3ee1c96c43d382c0925641d91878218
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/333560
Auto-Submit: Jake Macdonald <jakemac@google.com>
Commit-Queue: Daco Harkes <dacoharkes@google.com>
Commit-Queue: Jake Macdonald <jakemac@google.com>
Reviewed-by: Daco Harkes <dacoharkes@google.com>

samples/ffi/http/pubspec.yaml

@@ -3,7 +3,7 @@ version: 0.0.1
 publish_to: none
 
 environment:
-  sdk: '>=3.0.0 <4.0.0'
+  sdk: '>=3.1.0 <4.0.0'
 
 dependencies:
   ffi: ^2.1.0

samples/ffi/http/test/http_test.dart

@@ -3,12 +3,10 @@
 // BSD-style license that can be found in the LICENSE file.
 
 import 'dart:async';
-import 'dart:io';
 
-import 'package:ffi/ffi.dart';
 import 'package:test/test.dart';
 
-import '../lib/http.dart';
+import 'package:http/http.dart';
 
 Future<void> main() async {
   test('httpGet', () async {