Allow zero analyzer args in build mode.

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_cli/src/driver.dart';
 import 'package:args/args.dart';
@@ skipping 490 matching lines @@
               'No source files expected in the persistent worker mode.');
           _showUsage(parser);
           exitHandler(15);
           return null; // Only reachable in testing.
         }
       } else if (results['version']) {
         outSink.write('$_binaryName version ${_getVersion()}');
         exitHandler(0);
         return null; // Only reachable in testing.
       } else {
-        if (results.rest.isEmpty) {
+        if (results.rest.isEmpty && !results['build-mode']) {
           _showUsage(parser);
           exitHandler(15);
           return null; // Only reachable in testing.
         }
       }
       return new CommandLineOptions._fromArgs(results, definedVariables);
     } on FormatException catch (e) {
       errorSink.writeln(e.message);
       _showUsage(parser);
       exitHandler(15);
@@ skipping 145 matching lines @@
 
   int _getNextFlagIndex(args, i) {
     for (; i < args.length; ++i) {
       if (args[i].startsWith('--')) {
         return i;
       }
     }
     return i;
   }
 }