Make invocation namedArguments unmodifiable.

runtime/lib/invocation_mirror_patch.dart

Index: runtime/lib/invocation_mirror_patch.dart
diff --git a/runtime/lib/invocation_mirror_patch.dart b/runtime/lib/invocation_mirror_patch.dart
index e06c531c58de18b44c4e46400067ab28a52157ff..cc820cbb0a27dd7c52375d5961a948588bb1ac93 100644
--- a/runtime/lib/invocation_mirror_patch.dart
+++ b/runtime/lib/invocation_mirror_patch.dart
@@ -89,6 +89,7 @@ class _InvocationMirror implements Invocation {
         _namedArguments[new internal.Symbol.unvalidated(arg_name)] =
             arg_value;
       }
+      _namedArguments = new Map.unmodifiable(_namedArguments);
     }
     return _namedArguments;
   }