Add VersionConstraint.difference().

test/version_test.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 'package:test/test.dart';
 
 import 'package:pub_semver/pub_semver.dart';
 
 import 'utils.dart';
 
@@ skipping 184 matching lines @@
     });
 
     test("with a range allows both the range and the version if the range "
         "doesn't contain the version", () {
       var result = v123.union(new VersionRange(min: v003, max: v114));
       expect(result, allows(v123));
       expect(result, allows(v010));
     });
   });
 
+  group('difference()', () {
+    test("with the same version returns an empty constraint", () {
+      expect(v123.difference(v123), isEmpty);
+    });
+
+    test("with a different version returns the original version", () {
+      expect(v123.difference(v080), equals(v123));
+    });
+
+    test("returns an empty constraint with a range that contains the version",
+        () {
+      expect(v123.difference(new VersionRange(min: v114, max: v124)), isEmpty);
+    });
+
+    test("returns the version constraint with a range that doesn't contain it",
+        () {
+      expect(v123.difference(new VersionRange(min: v140, max: v300)),
+          equals(v123));
+    });
+  });
+
   test('isEmpty', () {
     expect(v123.isEmpty, isFalse);
   });
 
   test('nextMajor', () {
     expect(v123.nextMajor, equals(v200));
     expect(v114.nextMajor, equals(v200));
     expect(v200.nextMajor, equals(v300));
 
     // Ignores pre-release if not on a major version.
@@ skipping 84 matching lines @@
       expect(new Version(1, 2, 3, pre: 'pre', build: 'bui').toString(),
           equals('1.2.3-pre+bui'));
     });
 
     test('preserves leading zeroes', () {
       expect(new Version.parse('001.02.0003-01.dev+pre.002').toString(),
           equals('001.02.0003-01.dev+pre.002'));
     });
   });
 }