Swap getUsage() for get usage in args package

pkg/args/lib/src/arg_parser.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.
 
 library args.src.arg_parser;
 
 import 'package:collection/wrappers.dart';
 
 import 'arg_results.dart';
 import 'option.dart';
@@ skipping 97 matching lines @@
   }
 
   /// Parses [args], a list of command-line arguments, matches them against the
   /// flags and options defined by this parser, and returns the result.
   ArgResults parse(List<String> args) =>
       new Parser(null, this, args.toList(), null, null).parse();
 
   /// Generates a string displaying usage information for the defined options.
   ///
   /// This is basically the help text shown on the command line.
+  @Deprecated("Replaced with get usage. getUsage() will be removed in args 1.0")
   String getUsage() => new Usage(this).generate();
 
+  /// Generates a string displaying usage information for the defined options.
+  ///
+  /// This is basically the help text shown on the command line.
+  String get usage => new Usage(this).generate();
+
   /// Get the default value for an option. Useful after parsing to test if the
   /// user specified something other than the default.
   getDefault(String option) {
     if (!options.containsKey(option)) {
       throw new ArgumentError('No option named $option');
     }
     return options[option].defaultValue;
   }
 
   /// Finds the option whose abbreviation is [abbr], or `null` if no option has
   /// that abbreviation.
   Option findByAbbreviation(String abbr) {
     return options.values.firstWhere((option) => option.abbreviation == abbr,
         orElse: () => null);
   }
 }