Use pub_semver package in pub.

sdk/lib/_internal/pub_generated/lib/src/preprocess.dart

 library pub.preprocess;
+import 'package:pub_semver/pub_semver.dart';
 import 'package:string_scanner/string_scanner.dart';
-import 'version.dart';
 String preprocess(String input, Map<String, Version> versions, sourceUrl) {
   if (!input.contains(new RegExp(r"^//[>#]", multiLine: true))) return input;
   return new _Preprocessor(input, versions, sourceUrl).run();
 }
 class _Preprocessor {
   final StringScanner _scanner;
   final Map<String, Version> _versions;
   final _buffer = new StringBuffer();
   _Preprocessor(String input, this._versions, sourceUrl)
       : _scanner = new StringScanner(input, sourceUrl: sourceUrl);
@@ skipping 53 matching lines @@
       } else {
         _emitText();
       }
       _scanner.expect("//#");
       _scanner.scan(new RegExp(r"[ \t]*"));
     }
     _scanner.expect("end");
     if (!_scanner.isDone) _scanner.expect("\n");
   }
 }