Use pub_semver package in pub.

sdk/lib/_internal/pub/test/pubspec_test.dart

 // Copyright (c) 2012, 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.
 
 library pubspec_test;
 
 import 'dart:async';
 
+import 'package:pub_semver/pub_semver.dart';
 import 'package:unittest/unittest.dart';
 
 import '../lib/src/package.dart';
 import '../lib/src/pubspec.dart';
 import '../lib/src/source.dart';
 import '../lib/src/source_registry.dart';
-import '../lib/src/version.dart';
 import 'test_pub.dart';
 
 class MockSource extends Source {
   final String name = "mock";
 
   Future<Pubspec> doDescribe(PackageId id) => throw new UnsupportedError(
       "Cannot describe mock packages.");
 
   Future get(PackageId id, String symlink) => throw new UnsupportedError(
       "Cannot get a mock package.");
@@ skipping 466 matching lines @@
       test("uses the key if the value is null", () {
         var pubspec = new Pubspec.parse('''
 executables:
   command:
 ''', sources);
         expect(pubspec.executables['command'], equals('command'));
       });
     });
   });
 }