Correct --persistent_worker arg constraint check

pkg/analyzer_cli/lib/src/options.dart

 // Copyright (c) 2015, 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 analyzer_cli.src.options;
 
 import 'dart:io';
 
 import 'package:analyzer/file_system/physical_file_system.dart';
 import 'package:analyzer/src/command_line/arguments.dart';
@@ skipping 438 matching lines @@
           hide: hide);
 
     try {
       if (args.contains('--$ignoreUnrecognizedFlagsFlag')) {
         args = filterUnknownArguments(args, parser);
       }
       ArgResults results = parser.parse(args);
 
       // Persistent worker.
       if (args.contains('--persistent_worker')) {
-        bool validArgs;
-        if (!args.contains('--build-mode')) {
-          validArgs = false;
-        } else if (args.length == 2) {
-          validArgs = true;
-        } else if (args.length == 4 && args.contains('--dart-sdk')) {
-          validArgs = true;
-        } else {
-          validArgs = false;
-        }
-        if (!validArgs) {
+        bool hasBuildMode = args.contains('--build-mode');
+        bool onlyDartSdkArg = args.length == 2 ||
+            (args.length == 3 && args.any((a) => a.startsWith('--dart-sdk'))) ||
+            (args.length == 4 && args.contains('--dart-sdk'));
+        if (!(hasBuildMode && onlyDartSdkArg)) {
           printAndFail('The --persistent_worker flag should be used with and '
               'only with the --build-mode flag, and possibly the --dart-sdk '
               'option. Got: $args');
           return null; // Only reachable in testing.
         }
         return new CommandLineOptions._fromArgs(results);
       }
 
       // Help requests.
       if (results['help']) {
@@ skipping 42 matching lines @@
         'Usage: $_binaryName [options...] <directory or list of files>');
     errorSink.writeln('');
     errorSink.writeln(parser.usage);
     errorSink.writeln('');
     errorSink.writeln('''
 Run "dartanalyzer -h -v" for verbose help output, including less commonly used options.
 For more information, see http://www.dartlang.org/tools/analyzer.
 ''');
   }
 }