Allow zero analyzer args in build mode.

pkg/analyzer_cli/test/options_test.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.test.options;
 
 import 'dart:io';
 
 import 'package:analyzer_cli/src/driver.dart';
 import 'package:analyzer_cli/src/options.dart';
@@ skipping 239 matching lines @@
     ]);
     expect(options.buildMode, isTrue);
     expect(options.buildAnalysisOutput, '//path/to/output.analysis');
   }
 
   test_buildMode() {
     _parse(['--build-mode', 'package:p/foo.dart|/path/to/p/lib/foo.dart']);
     expect(options.buildMode, isTrue);
   }
 
+  test_buildMode_allowsEmptyFileList() {
+    _parse(['--build-mode']);
+    expect(options.buildMode, isTrue);
+    expect(options.sourceFiles, isEmpty);
+  }
+
   test_buildSummaryFallback() {
     _parse([
       '--build-mode',
       '--build-summary-output=//path/to/output.sum',
       '--build-summary-fallback',
       'package:p/foo.dart|/path/to/p/lib/foo.dart'
     ]);
     expect(options.buildMode, isTrue);
     expect(options.buildSummaryFallback, isTrue);
   }
@@ skipping 49 matching lines @@
       'package:p/foo.dart|/path/to/p/lib/foo.dart'
     ]);
     expect(options.buildMode, isTrue);
     expect(options.buildSuppressExitCode, isTrue);
   }
 
   void _parse(List<String> args) {
     options = CommandLineOptions.parse(args);
   }
 }