Switch analysis_server to use 'package:test'.

pkg/analysis_server/benchmark/perf/benchmark_scenario.dart

 // Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library server.performance.scenarios;
 
 import 'dart:async';
 import 'dart:io';
 import 'dart:math';
 
 import 'package:analysis_server/plugin/protocol/protocol.dart';
-import 'package:unittest/unittest.dart';
+import 'package:test/test.dart';
 
 import 'performance_tests.dart';
 
 void printBenchmarkResults(String id, String description, List<int> times) {
   int minTime = times.fold(1 << 20, min);
   String now = new DateTime.now().toUtc().toIso8601String();
   print('$now ========== $id');
   print('times: $times');
   print('min_time: $minTime');
   print(description.trim());
@@ skipping 271 matching lines @@
   final String replaceWith;
 
   FileChange({this.afterStr, this.afterStrBack: 0, this.insertStr, this.replaceWhat, this.replaceWith}) {
     if (afterStr != null) {
       expect(insertStr, isNotNull, reason: 'insertStr');
     } else if (replaceWhat != null) {
       expect(replaceWith, isNotNull, reason: 'replaceWith');
     }
   }
 }