return Version.none instead of null

lib/pub_cache.dart

diff --git a/lib/pub_cache.dart b/lib/pub_cache.dart
index 3f980e5..26ab298 100644
--- a/lib/pub_cache.dart
+++ b/lib/pub_cache.dart
@@ -143,7 +143,7 @@
   /// The version of the application and of the defining package.
   Version get version {
     PackageRef ref = getDefiningPackageRef();
-    return ref == null ? null : ref.version;
+    return ref == null ? Version.none : ref.version;
   }
 
   /// Return the reference to the defining package. This is the package that

lib/src/impl.dart

diff --git a/lib/src/impl.dart b/lib/src/impl.dart
index 3a23de1..59bad03 100644
--- a/lib/src/impl.dart
+++ b/lib/src/impl.dart
@@ -118,11 +118,13 @@
     }
 
     // Parse the version.
+    _version = Version.none;
     File f = new File(path.join(directory.path, 'pubspec.yaml'));
     if (f.existsSync()) {
       Map pubspec = yaml.loadYaml(f.readAsStringSync());
-      _version = pubspec.containsKey('version')
-          ? new Version.parse(pubspec['version']) : Version.none;
+      if (pubspec.containsKey('version')) {
+        _version = new Version.parse(pubspec['version']);
+      }
     }
   }