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..c7b0d1d45f342edf0bd3aa04bfd136c44be3125b 100644
--- a/utils/pub/version.dart
+++ b/utils/pub/version.dart
@@ -9,8 +9,12 @@
  */
 #library('pub_version');
 
+#import('utils.dart');
+
 /** 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 +108,8 @@ class Version implements Comparable, VersionConstraint {
     return 0;
   }
 
+  int hashCode() => toString().hashCode();
+
   String toString() {
     var buffer = new StringBuffer();
     buffer.add('$major.$minor.$patch');
@@ -201,12 +207,3 @@ class VersionRange implements VersionConstraint {
     return true;
   }
 }
-
-/** 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;
-}