Add more set-like version constraint operations.

lib/src/utils.dart

+// Copyright (c) 2015, 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.src.utils;
+
+import 'version_range.dart';
+
+/// Returns whether [range1] is immediately next to, but not overlapping,
+/// [range2].
+bool areAdjacent(VersionRange range1, VersionRange range2) {
+  if (range1.max != range2.min) return false;
+
+  return (range1.includeMax && !range2.includeMin) ||
+      (!range1.includeMax && range2.includeMin);
+}
+
+/// A [Comparator] that compares the maximum versions of [range1] and [range2].
+int compareMax(VersionRange range1, VersionRange range2) {
+  if (range1.max < range2.max) return -1;
+  if (range1.max > range2.max) return 1;
+
+  if (!range1.includeMax && range2.includeMax) return -1;
+  if (range1.includeMax && !range2.includeMax) return 1;
+  return 0;
+}