Add VersionConstraint.difference().

lib/src/version.dart

 // Copyright (c) 2014, 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.
 
 import 'dart:math' as math;
 
 import 'package:collection/collection.dart';
 
 import 'patterns.dart';
 import 'version_constraint.dart';
@@ skipping 249 matching lines @@
       if (other.max == this) {
         return new VersionRange(
             min: other.min, max: other.max,
             includeMin: other.includeMin, includeMax: true);
       }
     }
 
     return new VersionConstraint.unionOf([this, other]);
   }
 
+  VersionConstraint difference(VersionConstraint other) =>
+      other.allows(this) ? VersionConstraint.empty : this;
+
   int compareTo(VersionRange other) {
     if (other is Version) {
       if (major != other.major) return major.compareTo(other.major);
       if (minor != other.minor) return minor.compareTo(other.minor);
       if (patch != other.patch) return patch.compareTo(other.patch);
 
       // Pre-releases always come before no pre-release string.
       if (!isPreRelease && other.isPreRelease) return 1;
       if (!other.isPreRelease && isPreRelease) return -1;
 
@@ skipping 42 matching lines @@
           // Compare two strings.
           return aPart.compareTo(bPart);
         }
       }
     }
 
     // The lists are entirely equal.
     return 0;
   }
 }