Use pub_semver package in pub.

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

 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.");
   Future<String> getDirectory(PackageId id) =>
       throw new UnsupportedError("Cannot get the directory for mock packages.");
   dynamic parseDescription(String filePath, description, {bool fromLockFile:
@@ skipping 412 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'));
       });
     });
   });
 }