UsageException isn't called UsageError.

lib/command_runner.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 'dart:async';
 import 'dart:collection';
 import 'dart:math' as math;
 
 import 'src/arg_parser.dart';
 import 'src/arg_results.dart';
@@ skipping 80 matching lines @@
     for (var name in names) {
       _commands[name] = command;
       argParser.addCommand(name, command.argParser);
     }
     command._runner = this;
   }
 
   /// Parses [args] and invokes [Command.run] on the chosen command.
   ///
   /// This always returns a [Future] in case the command is asynchronous. The
-  /// [Future] will throw a [UsageError] if [args] was invalid.
+  /// [Future] will throw a [UsageException] if [args] was invalid.
   Future run(Iterable<String> args) =>
       new Future.sync(() => runCommand(parse(args)));
 
   /// Parses [args] and returns the result, converting a [FormatException] to a
   /// [UsageException].
   ///
   /// This is notionally a protected method. It may be overridden or called from
   /// subclasses, but it shouldn't be called externally.
   ArgResults parse(Iterable<String> args) {
     try {
@@ skipping 271 matching lines @@
 
     for (var line in lines.skip(1)) {
       buffer.writeln();
       buffer.write(' ' * (length + 5));
       buffer.write(line);
     }
   }
 
   return buffer.toString();
 }