Support "pub downgrade".

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 '../lib/src/exit_codes.dart' as exit_codes;
 import 'test_pub.dart';
@@ skipping 10 matching lines @@
         --verbosity       Control output verbosity.
 
               [all]       Show all output including internal tracing messages.
               [io]        Also show IO operations.
               [normal]    Show errors, warnings, and user messages.
               [solver]    Show steps during version resolution.
 
     -v, --verbose         Shortcut for "--verbosity=all".
 
     Available commands:
-      build      Apply transformers to build a package.
-      cache      Work with the system cache.
-      deps       Print package dependencies.
-      get        Get the current package's dependencies.
-      global     Work with global packages.
-      help       Display help information for Pub.
-      publish    Publish the current package to pub.dartlang.org.
-      run        Run an executable from a package.
-      serve      Run a local web development server.
-      upgrade    Upgrade the current package's dependencies to latest versions.
-      uploader   Manage uploaders for a package on pub.dartlang.org.
-      version    Print pub version.
+      build       Apply transformers to build a package.
+      cache       Work with the system cache.
+      deps        Print package dependencies.
+      downgrade   Downgrade the current package's dependencies to oldest versions.
+      get         Get the current package's dependencies.
+      global      Work with global packages.
+      help        Display help information for Pub.
+      publish     Publish the current package to pub.dartlang.org.
+      run         Run an executable from a package.
+      serve       Run a local web development server.
+      upgrade     Upgrade the current package's dependencies to latest versions.
+      uploader    Manage uploaders for a package on pub.dartlang.org.
+      version     Print pub version.
 
     Run "pub help [command]" for more information about a command.
     See http://dartlang.org/tools/pub for detailed documentation.
     """;
 
 final VERSION_STRING = '''
     Pub 0.1.2+3
     ''';
 
 main() {
@@ skipping 67 matching lines @@
   integration('running pub with just --version displays version', () {
     schedulePub(args: ['--version'], output: VERSION_STRING);
   });
 
   integration('an unknown command displays an error message', () {
     schedulePub(args: ['quylthulg'],
         error: '''
         Could not find a command named "quylthulg".
 
         Available commands:
-          build      Apply transformers to build a package.
-          cache      Work with the system cache.
-          deps       Print package dependencies.
-          get        Get the current package's dependencies.
-          global     Work with global packages.
-          help       Display help information for Pub.
-          publish    Publish the current package to pub.dartlang.org.
-          run        Run an executable from a package.
-          serve      Run a local web development server.
-          upgrade    Upgrade the current package's dependencies to latest versions.
-          uploader   Manage uploaders for a package on pub.dartlang.org.
-          version    Print pub version.
+          build       Apply transformers to build a package.
+          cache       Work with the system cache.
+          deps        Print package dependencies.
+          downgrade   Downgrade the current package's dependencies to oldest versions.
+          get         Get the current package's dependencies.
+          global      Work with global packages.
+          help        Display help information for Pub.
+          publish     Publish the current package to pub.dartlang.org.
+          run         Run an executable from a package.
+          serve       Run a local web development server.
+          upgrade     Upgrade the current package's dependencies to latest versions.
+          uploader    Manage uploaders for a package on pub.dartlang.org.
+          version     Print pub version.
         ''',
         exitCode: exit_codes.USAGE);
   });
 
   integration('an unknown subcommand displays an error message', () {
     schedulePub(args: ['cache', 'quylthulg'],
         error: '''
         Could not find a subcommand named "quylthulg" for "pub cache".
 
         Usage: pub cache <subcommand>
@@ skipping 138 matching lines @@
             Run "pub help" to see global options.
             ''');
     });
 
     integration('an unknown help command displays an error message', () {
       schedulePub(args: ['help', 'quylthulg'],
           error: '''
             Could not find a command named "quylthulg".
 
             Available commands:
-              build      Apply transformers to build a package.
-              cache      Work with the system cache.
-              deps       Print package dependencies.
-              get        Get the current package's dependencies.
-              global     Work with global packages.
-              help       Display help information for Pub.
-              publish    Publish the current package to pub.dartlang.org.
-              run        Run an executable from a package.
-              serve      Run a local web development server.
-              upgrade    Upgrade the current package's dependencies to latest versions.
-              uploader   Manage uploaders for a package on pub.dartlang.org.
-              version    Print pub version.
+              build       Apply transformers to build a package.
+              cache       Work with the system cache.
+              deps        Print package dependencies.
+              downgrade   Downgrade the current package's dependencies to oldest versions.
+              get         Get the current package's dependencies.
+              global      Work with global packages.
+              help        Display help information for Pub.
+              publish     Publish the current package to pub.dartlang.org.
+              run         Run an executable from a package.
+              serve       Run a local web development server.
+              upgrade     Upgrade the current package's dependencies to latest versions.
+              uploader    Manage uploaders for a package on pub.dartlang.org.
+              version     Print pub version.
             ''',
             exitCode: exit_codes.USAGE);
     });
 
     integration('an unknown help subcommand displays an error message', () {
       schedulePub(args: ['help', 'cache', 'quylthulg'],
           error: '''
             Could not find a subcommand named "quylthulg" for "pub cache".
 
             Usage: pub cache <subcommand>
@@ skipping 22 matching lines @@
             exitCode: exit_codes.USAGE);
     });
   });
 
   group('version', () {
     integration('displays the current version', () {
       schedulePub(args: ['version'], output: VERSION_STRING);
     });
   });
 }