Make "pub help" show global help.

sdk/lib/_internal/pub/test/pub_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 pub_tests;
 
 import 'package:scheduled_test/scheduled_test.dart';
 
 import 'test_pub.dart';
 
@@ skipping 110 matching lines @@
         Usage: pub version
          -h, --help    Print usage information for this command.
         ''',
         error: '''
         Command "version" does not take any arguments.
         ''',
         exitCode: 64);
   });
 
   group('help', () {
+    integration('shows global help if no command is given', () {
+      schedulePub(args: ['help'], output: USAGE_STRING);
+    });
+
     integration('shows help for a command', () {
       schedulePub(args: ['help', 'get'],
           output: '''
             Get the current package's dependencies.
 
             Usage: pub get
             -h, --help            Print usage information for this command.
                 --[no-]offline    Use cached packages instead of accessing the network.
             ''');
     });
@@ skipping 22 matching lines @@
     });
 
   });
 
   group('version', () {
     integration('displays the current version', () {
       schedulePub(args: ['version'], output: VERSION_STRING);
     });
   });
 }