Switch analyzer_cli to 'package:test'.

pkg/analyzer_cli/test/package_prefix_test.dart

 import 'dart:io' show exitCode;
 
 import 'package:analyzer_cli/src/driver.dart' show Driver, outSink, errorSink;
 import 'package:analyzer_cli/src/options.dart' show ExitHandler, exitHandler;
 import 'package:path/path.dart';
-import 'package:unittest/unittest.dart';
+import 'package:test/test.dart';
 
 import 'utils.dart' show testDirectory;
 
 main() {
   group('--x-package-warnings-prefix', () {
     _Runner runner;
 
     setUp(() {
       runner = new _Runner.setUp();
     });
@@ skipping 48 matching lines @@
     }
   }
 
   void tearDown() {
     outSink = _savedOutSink;
     errorSink = _savedErrorSink;
     exitCode = _savedExitCode;
     exitHandler = _savedExitHandler;
   }
 }