Fixes to get Swarm demo working again.

frog/value.dart

Index: frog/value.dart
diff --git a/frog/value.dart b/frog/value.dart
index 7205f78b1ce72a8409f7a1c9807ccbf935e9d4e4..3672a9d5c4f0e74ee672942b84c9c3825e22e44b 100644
--- a/frog/value.dart
+++ b/frog/value.dart
@@ -177,7 +177,11 @@ class Value {
     if (member == null && !isSuper && !isType) {
       member = context.findMembers(name);
       if (member == null && !isDynamic) {
-        world.warning('$name is not defined anywhere in the world.',
+        var where = 'the world';
+        if (name.startsWith('_')) {
+          where = 'library "${context.library.name}"';
+        }
+        world.warning('$name is not defined anywhere in $where.',
            node.span);
       }
     }