Added special case code for translating null element to work around a frog bug.

corelib/src/implementation/collections.dart

Index: corelib/src/implementation/collections.dart
diff --git a/corelib/src/implementation/collections.dart b/corelib/src/implementation/collections.dart
index 2bed4aa7daea028eb749ce42cdeb4a0311b89249..162f6422b197d77e7ee3e51f4b72f07982e33945 100644
--- a/corelib/src/implementation/collections.dart
+++ b/corelib/src/implementation/collections.dart
@@ -133,7 +133,7 @@ class Collections {
         Maps._emitMap(o, result, visiting);
       }
     } else { // o is neither a collection nor a map
-      result.add(o);
+      result.add(o == null ? 'null' : o);  // TODO(jjb): remove the null check
     }
   }