Exit with an error on unexpected command line arguments.

sdk/lib/_internal/pub/lib/src/command/cache.dart

 // Copyright (c) 2013, 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 pub.command.cache;
 
 import 'dart:async';
 import 'dart:convert';
 
 import '../command.dart';
 import '../exit_codes.dart' as exit_codes;
 import '../io.dart';
 import '../log.dart' as log;
 
 /// Handles the `cache` pub command.
 class CacheCommand extends PubCommand {
   String get description => "Inspect the system cache.";
   String get usage => 'pub cache list';
   bool get hidden => true;
   bool get requiresEntrypoint => false;
+  bool get takesArguments => true;
 
   Future onRun() {
+    // TODO(rnystrom): Use subcommand for "list".
     if (commandOptions.rest.length != 1) {
       log.error('The cache command expects one argument.');
       this.printUsage();
       return flushThenExit(exit_codes.USAGE);
     }
 
     if ((commandOptions.rest[0] != 'list')) {
       log.error('Unknown cache command "${commandOptions.rest[0]}".');
       this.printUsage();
       return flushThenExit(exit_codes.USAGE);
     }
 
     // TODO(keertip): Add flag to list packages from non default sources
     var packagesObj = <String, Map>{};
 
     for (var package in cache.sources.defaultSource.getCachedPackages()) {
 
       var packageInfo = packagesObj.putIfAbsent(package.name, () => {});
       packageInfo[package.version.toString()] = {'location': package.dir};
     }
 
     // TODO(keertip): Add support for non-JSON format
     // and check for --format flag
     log.message(JSON.encode({'packages': packagesObj}));
   }
 }