Swap getUsage() for get usage in args package

pkg/args/test/usage_test.dart

 // Copyright (c) 2012, 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 usage_test;
 
 import 'package:unittest/unittest.dart';
 import 'package:args/args.dart';
 
 void main() {
-  group('ArgParser.getUsage()', () {
+  group('ArgParser.usage', () {
     test('negatable flags show "no-" in title', () {
       var parser = new ArgParser();
       parser.addFlag('mode', help: 'The mode');
 
       validateUsage(parser,
           '''
           --[no-]mode    The mode
           ''');
     });
 
@@ skipping 196 matching lines @@
           '''
           --[no-]first    The first flag
           --[no-]third    The third flag
           ''');
     });
   });
 }
 
 void validateUsage(ArgParser parser, String expected) {
   expected = unindentString(expected);
-  expect(parser.getUsage(), equals(expected));
+  expect(parser.usage, equals(expected));
 }
 
 // TODO(rnystrom): Replace one in test_utils.
 String unindentString(String text) {
   var lines = text.split('\n');
 
   // Count the indentation of the last line.
   var whitespace = new RegExp('^ *');
   var indent = whitespace.firstMatch(lines[lines.length - 1])[0].length;
 
@@ skipping 16 matching lines @@
         throw new ArgumentError(
             'Line "$line" does not have enough indentation.');
       }
 
       lines[i] = line.substring(indent);
     }
   }
 
   return lines.join('\n');
 }