First pass at version constraint solver.

utils/pub/pubspec.dart

 // Copyright (c) 2012, 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('pubspec');
 
 #import('package.dart');
 #import('source.dart');
 #import('source_registry.dart');
 #import('utils.dart');
@@ skipping 41 matching lines @@
 
     if (parsedPubspec.containsKey('dependencies')) {
       var dependencyEntries = parsedPubspec['dependencies'];
       if (dependencyEntries is! Map ||
           dependencyEntries.getKeys().some((e) => e is! String)) {
         throw new FormatException(
             'The pubspec dependencies must be a map of package names.');
       }
 
       dependencyEntries.forEach((name, spec) {
-        var description, source;
+        var description, source, versionConstraint;
         if (spec == null) {
           description = name;
           source = sources.defaultSource;
         } else if (spec is String) {
           description = name;
           source = sources.defaultSource;
-          // TODO(rnystrom): Support parsing version constraints like
-          // ">= 2.0.0 < 3.1.3".
-          version = new Version.parse(spec);
+          versionConstraint = new VersionConstraint.parse(spec);
         } else if (spec is Map) {
           if (spec.containsKey('version')) {
-            // TODO(rnystrom): Support parsing version constraints like
-            // ">= 2.0.0 < 3.1.3".
-            version = new Version.parse(spec.remove('version'));
+            versionConstraint = new VersionConstraint.parse(
+                spec.remove('version'));
           }
 
           var sourceNames = spec.getKeys();
           if (sourceNames.length > 1) {
             throw new FormatException(
                 'Dependency $name may only have one source: $sourceNames.');
           }
 
           var sourceName = only(sourceNames);
           if (sourceName is! String) {
             throw new FormatException(
                 'Source name $sourceName must be a string.');
           }
 
           source = sources[sourceName];
           description = spec[sourceName];
         } else {
           throw new FormatException(
               'Dependency specification $spec must be a string or a mapping.');
         }
 
         source.validateDescription(description);
 
         dependencies.add(new PackageRef(
-            name, source, Version.none, description));
+            name, source, versionConstraint, description));
       });
     }
 
     return new Pubspec._(version, dependencies);
   }
 }