Make VersionRange implement Comparable.

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.
 
 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.max == null) {
+    if (range2.max == null) return 0;
+    return 1;
+  } else if (range2.max == null) {
+    return -1;
+  }
 
-  if (!range1.includeMax && range2.includeMax) return -1;
-  if (range1.includeMax && !range2.includeMax) return 1;
+  var result = range1.max.compareTo(range2.max);
+  if (result != 0) return result;
+  if (range1.includeMax != range2.includeMax) return range1.includeMax ? 1 : -1;
   return 0;
 }