Get codebase ready for actually supporting versioning.

utils/pub/version.dart

Index: utils/pub/version.dart
diff --git a/utils/pub/version.dart b/utils/pub/version.dart
index 061c31951215d568166f189454f6b33b2192c04b..89c455d6bb8383f66b24a8fa7a6a4f154df50e8f 100644
--- a/utils/pub/version.dart
+++ b/utils/pub/version.dart
@@ -10,7 +10,9 @@
 #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.
@@ -104,6 +106,8 @@ class Version implements Comparable, VersionConstraint {
     return 0;
   }
 
+  int hashCode() => toString().hashCode();
+
   String toString() {
     var buffer = new StringBuffer();
     buffer.add('$major.$minor.$patch');