Correctly handle case where we have a library outside of a package.

pkg/docgen/lib/src/models/library.dart

Index: pkg/docgen/lib/src/models/library.dart
diff --git a/pkg/docgen/lib/src/models/library.dart b/pkg/docgen/lib/src/models/library.dart
index 826536fb1bfe86574e392c8774d3a4e22445c330..ccd12a22b7c07773298be38c2ba46bb35415272a 100644
--- a/pkg/docgen/lib/src/models/library.dart
+++ b/pkg/docgen/lib/src/models/library.dart
@@ -142,6 +142,8 @@ class Library extends Indexable {
     if (packageIntro != null) {
       map['packageIntro'] = packageIntro;
     }
+    var version = packageVersion(mirror);
+    if (version != '' && version != null) map['version'] = version;
     return map;
   }