Use pub_semver package in pub.

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

 library pub.barback;
 import 'package:barback/barback.dart';
 import 'package:path/path.dart' as p;
-import 'version.dart';
+import 'package:pub_semver/pub_semver.dart';
 final pubConstraints = {
   "barback": new VersionConstraint.parse(">=0.13.0 <0.15.3"),
   "source_span": new VersionConstraint.parse(">=1.0.0 <2.0.0"),
   "stack_trace": new VersionConstraint.parse(">=0.9.1 <2.0.0")
 };
 Uri idToPackageUri(AssetId id) {
   if (!id.path.startsWith('lib/')) {
     throw new ArgumentError("Asset id $id doesn't identify a library.");
   }
   return new Uri(
       scheme: 'package',
       path: p.url.join(id.package, id.path.replaceFirst('lib/', '')));
 }
 AssetId packagesUrlToId(Uri url) {
   var parts = p.url.split(url.path);
   if (parts.isNotEmpty && parts.first == "/") parts = parts.skip(1).toList();
   if (parts.isEmpty) return null;
   var index = parts.indexOf("packages");
   if (index == -1) return null;
   if (parts.length <= index + 1) {
     throw new FormatException(
         'Invalid URL path "${url.path}". Expected package name ' 'after "packages".');
   }
   var package = parts[index + 1];
   var assetPath = p.url.join("lib", p.url.joinAll(parts.skip(index + 2)));
   return new AssetId(package, assetPath);
 }