Use pub_semver package in pub.

sdk/lib/_internal/pub/lib/src/preprocess.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.
 
 library pub.preprocess;
 
+import 'package:pub_semver/pub_semver.dart';
 import 'package:string_scanner/string_scanner.dart';
 
-import 'version.dart';
-
 /// Runs a simple preprocessor over [input] to remove sections that are
 /// incompatible with the available barback version.
 ///
 /// [versions] are the available versions of each installed package, and
 /// [sourceUrl] is a [String] or [Uri] indicating where [input] came from. It's
 /// used for error reporting.
 ///
 /// For the most part, the preprocessor leaves text in the source document
 /// alone. However, it handles two types of lines specially. Lines that begin
 /// with `//>` are uncommented by the preprocessor, and lines that begin with
@@ skipping 114 matching lines @@
         _emitText();
       }
       _scanner.expect("//#");
       _scanner.scan(new RegExp(r"[ \t]*"));
     }
 
     _scanner.expect("end");
     if (!_scanner.isDone) _scanner.expect("\n");
   }
 }