Harvest from pub. Mostly a straight copy, except:

lib/pub_semver.dart

Index: lib/pub_semver.dart
diff --git a/lib/pub_semver.dart b/lib/pub_semver.dart
new file mode 100644
index 0000000000000000000000000000000000000000..436e226005984fbce84eb7a1ba9f3fe44e3ffd75
--- /dev/null
+++ b/lib/pub_semver.dart
@@ -0,0 +1,9 @@
+// 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_semver;
+
+export 'src/version.dart';
+export 'src/version_constraint.dart';
+export 'src/version_range.dart';