Add VersionConstraint.difference().

lib/src/version_constraint.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 'patterns.dart';
 import 'version.dart';
 import 'version_range.dart';
 import 'version_union.dart';
 import 'utils.dart';
 
@@ skipping 218 matching lines @@
   /// allows.
   bool allowsAny(VersionConstraint other);
 
   /// Creates a new [VersionConstraint] that only allows [Version]s allowed by
   /// both this and [other].
   VersionConstraint intersect(VersionConstraint other);
 
   /// Creates a new [VersionConstraint] that allows [Versions]s allowed by
   /// either this or [other].
   VersionConstraint union(VersionConstraint other);
+
+  /// Creates a new [VersionConstraint] that allows [Version]s allowed by this

[Bob Nystrom, 2016/06/08 22:51:06]

Technically, these don't always create *new* constraints. How about just
"Returns a [VersionConstraint]..."

[nweiz, 2016/06/08 23:39:02]

Done.

+  /// but not [other].
+  VersionConstraint difference(VersionConstraint other);
 }
 
 class _EmptyVersion implements VersionConstraint {
   const _EmptyVersion();
 
   bool get isEmpty => true;
   bool get isAny => false;
   bool allows(Version other) => false;
   bool allowsAll(VersionConstraint other) => other.isEmpty;
   bool allowsAny(VersionConstraint other) => false;
   VersionConstraint intersect(VersionConstraint other) => this;
   VersionConstraint union(VersionConstraint other) => other;
+  VersionConstraint difference(VersionConstraint other) => this;
   String toString() => '<empty>';
 }
 
 class _CompatibleWithVersionRange extends VersionRange {
   _CompatibleWithVersionRange(Version version) : super(
       min: version, includeMin: true,
       max: version.nextBreaking, includeMax: false);
 
   String toString() => '^$min';
 }