Get codebase ready for actually supporting versioning.

utils/pub/version.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.
 
 /**
  * Handles version numbers, following the [Semantic Versioning][semver] spec.
  *
  * [semver]: http://semver.org/
  */
 #library('pub_version');
 
 /** A parsed semantic version number. */
-class Version implements Comparable, VersionConstraint {
+class Version implements Comparable, Hashable, VersionConstraint {
+  static get none() => new Version(0, 0, 0);
+
   static final _PARSE_REGEX = const RegExp(
       @'^'                                        // Start at beginning.
       @'(\d+).(\d+).(\d+)'                        // Version number.
       @'(-([0-9A-Za-z-]+(\.[0-9A-Za-z-]+)*))?'  // Pre-release.
       @'(\+([0-9A-Za-z-]+(\.[0-9A-Za-z-]+)*))?' // Build.
       @'$');                                      // Consume entire string.
 
   /** The major version number: "1" in "1.2.3". */
   final int major;
 
@@ skipping 73 matching lines @@
       // Builds always come after no build string.
       if (build == null) return -1;
       if (other.build == null) return 1;
 
       return _compareStrings(build, other.build);
     }
 
     return 0;
   }
 
+  int hashCode() => toString().hashCode();
+
   String toString() {
     var buffer = new StringBuffer();
     buffer.add('$major.$minor.$patch');
     if (preRelease != null) buffer.add('-$preRelease');
     if (build != null) buffer.add('+$build');
     return buffer.toString();
   }
 
   /**
    * Compares the string part of two versions. This is used for the pre-release
@@ skipping 86 matching lines @@
 }
 
 /** Thrown by [Version.parse()] if the argument isn't a valid version string. */
 class FormatException implements Exception {
   final String message;
 
   FormatException(this.message);
 
   String toString() => message;
 }