Use backtracking when solving dependency constraints.

utils/pub/pub.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.
 
 /// The main entrypoint for the pub command line application.
 library pub;
 
 import 'dart:async';
 import 'dart:io';
 import 'dart:math';
@@ skipping 46 matching lines @@
 ArgParser get pubArgParser {
   var parser = new ArgParser();
   parser.addFlag('help', abbr: 'h', negatable: false,
       help: 'Print this usage information.');
   parser.addFlag('version', negatable: false,
       help: 'Print pub version.');
   parser.addFlag('trace',
        help: 'Print debugging information when an error occurs.');
   parser.addOption('verbosity',
       help: 'Control output verbosity.',
-      allowed: ['normal', 'io', 'all'],
+      allowed: ['normal', 'io', 'solver', 'all'],
       allowedHelp: {
-        'normal': 'Errors, warnings, and user messages are shown.',
-        'io':     'IO operations are also shown.',
-        'all':    'All output including internal tracing messages are shown.'
+        'normal': 'Show errors, warnings, and user messages.',
+        'io':     'Also show IO operations.',
+        'solver': 'Show steps during version resolution.',
+        'all':    'Show all output including internal tracing messages.'
       });
   parser.addFlag('verbose', abbr: 'v', negatable: false,
       help: 'Shortcut for "--verbosity=all"');
   return parser;
 }
 
 main() {
   var globalOptions;
   try {
     globalOptions = pubArgParser.parse(new Options().arguments);
@@ skipping 13 matching lines @@
     return;
   }
 
   if (globalOptions['trace']) {
     log.recordTranscript();
   }
 
   switch (globalOptions['verbosity']) {
     case 'normal': log.showNormal(); break;
     case 'io': log.showIO(); break;
+    case 'solver': log.showSolver(); break;
     case 'all': log.showAll(); break;
     default:
       // No specific verbosity given, so check for the shortcut.
       if (globalOptions['verbose']) {
         log.showAll();
       } else {
         log.showNormal();
       }
       break;
   }
@@ skipping 207 matching lines @@
     if (exception is HttpException || exception is HttpParserException ||
         exception is SocketIOException || exception is PubHttpException) {
       return exit_codes.UNAVAILABLE;
     } else if (exception is FormatException) {
       return exit_codes.DATA;
     } else {
       return 1;
     }
   }
 }