Add sample directories to the package config.

This allows the package imports in the tests to resolve.

Change-Id: Idadca8b55516a5b5d1511ebc38c298479aa8eda0
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/333662
Commit-Queue: Nate Bosch <nbosch@google.com>
Auto-Submit: Jake Macdonald <jakemac@google.com>
Reviewed-by: Nate Bosch <nbosch@google.com>

samples/ffi/http/pubspec.yaml

@@ -1,4 +1,4 @@
-name: http
+name: http_sample
 version: 0.0.1
 publish_to: none
 

samples/ffi/http/test/http_test.dart

@@ -6,7 +6,7 @@ import 'dart:async';
 
 import 'package:test/test.dart';
 
-import 'package:http/http.dart';
+import 'package:http_sample/http.dart';
 
 Future<void> main() async {
   test('httpGet', () async {