Add support for configuration presets.

lib/src/backend/metadata.dart

Index: lib/src/backend/metadata.dart
diff --git a/lib/src/backend/metadata.dart b/lib/src/backend/metadata.dart
index 6ff5412d6946aa5873230b9fdf8457e51ba3ac30..91bb54d07059b0a37c0efab8311cbd7af58fe899 100644
--- a/lib/src/backend/metadata.dart
+++ b/lib/src/backend/metadata.dart
@@ -140,6 +140,7 @@ class Metadata {
     // If there's no tag-specific metadata, or if none of it applies, just
     // return the metadata as-is.
     if (forTag == null || tags == null) return _unresolved();
+    tags = new Set.from(tags);
     forTag = new Map.from(forTag);
 
     // Otherwise, resolve the tag-specific components. Doing this eagerly means