Use pub_semver package in pub.

sdk/lib/_internal/pub/lib/src/sdk.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.
 
 /// Operations relative to the user's installed Dart SDK.
 library pub.sdk;
 
 import 'dart:io';
 
 import 'package:path/path.dart' as path;
+import 'package:pub_semver/pub_semver.dart';
 
 import 'io.dart';
-import 'version.dart';
 
 /// Gets the path to the root directory of the SDK.
 ///
 /// When running from the actual built SDK, this will be the SDK that contains
 /// the running Dart executable. When running from the repo, it will be the
 /// "sdk" directory in the Dart repository itself.
 final String rootDirectory =
     runningFromSdk ? _rootDirectory : path.join(repoRoot, "sdk");
 
 /// Gets the path to the root directory of the SDK, assuming that the currently
@@ skipping 41 matching lines @@
   var version = "$major.$minor.$patch";
   if (channel == "be") {
     // TODO(rnystrom): tools/utils.py includes the svn commit here. Should we?
     version += "-edge";
   } else if (channel == "dev") {
     version += "-dev.$prerelease.$prereleasePatch";
   }
 
   return new Version.parse(version);
 }