Make it strong mode clean.

lib/src/help_command.dart

 // Copyright (c) 2014, 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.
 
 import '../command_runner.dart';
 
 /// The built-in help command that's added to every [CommandRunner].
 ///
 /// This command displays help information for the various subcommands.
 class HelpCommand extends Command {
   final name = "help";
   String get description =>
       "Display help information for ${runner.executableName}.";
   String get invocation => "${runner.executableName} help [command]";
 
   void run() {
     // Show the default help if no command was specified.
     if (argResults.rest.isEmpty) {
       runner.printUsage();
       return;
     }
 
     // Walk the command tree to show help for the selected command or
     // subcommand.
     var commands = runner.commands;
-    var command = null;
+    Command command;
     var commandString = runner.executableName;
 
     for (var name in argResults.rest) {
       if (commands.isEmpty) {
         command.usageException(
             'Command "$commandString" does not expect a subcommand.');
       }
 
       if (commands[name] == null) {
         if (command == null) {
           runner.usageException('Could not find a command named "$name".');
         }
 
         command.usageException(
             'Could not find a subcommand named "$name" for "$commandString".');
       }
 
       command = commands[name];
       commands = command.subcommands;
       commandString += " $name";
     }
 
     command.printUsage();
   }
 }