Add on_os and on_platform fields.

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 2b38c08e0fb78c588418bc2389d568b1d96e548b..6ff5412d6946aa5873230b9fdf8457e51ba3ac30 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();
+    forTag = new Map.from(forTag);
 
     // Otherwise, resolve the tag-specific components. Doing this eagerly means
     // we only have to resolve suite- or group-level tags once, rather than