Extract testing utilities into a separate package.

pkg/analysis_server/test/package_uri_resolver_test.dart

Index: pkg/analysis_server/test/package_uri_resolver_test.dart
diff --git a/pkg/analysis_server/test/package_uri_resolver_test.dart b/pkg/analysis_server/test/package_uri_resolver_test.dart
index a445239dbef328ae0892eaf76fed597b52817807..d3ceebcdc5cb1b6629ec51063403491156e704ce 100644
--- a/pkg/analysis_server/test/package_uri_resolver_test.dart
+++ b/pkg/analysis_server/test/package_uri_resolver_test.dart
@@ -7,12 +7,12 @@ library test.resolver.package;
 import 'dart:collection';
 
 import 'package:analysis_server/src/package_uri_resolver.dart';
+import 'package:analysis_testing/reflective_tests.dart';
 import 'package:analyzer/file_system/file_system.dart';
 import 'package:analyzer/file_system/memory_file_system.dart';
 import 'package:analyzer/src/generated/source.dart';
 import 'package:unittest/unittest.dart';
 
-import 'reflective_tests.dart';
 
 main() {
   groupSep = ' | ';