anchor.diff:  import '../world.dart' show World;
   import 'cps_fragment.dart';
   import 'cps_ir_nodes.dart';
  +import 'cps_ir_nodes_sexpr.dart' show SExpressionStringifier;
   
   enum AbstractBool {
     True, False, Maybe, Nothing
  
positive.code: 
  import 'optimizers.dart';
  
  import '../common/names.dart' show
      Selectors;
  import '../compiler.dart' as dart2js show
      Compiler;
  import '../constants/constant_system.dart';
  
negative.code:     if (specializeFieldAccess(node)) return;
      if (specializeIndexableAccess(node)) return;
      if (specializeArrayAccess(node)) return;
      if (specializeSingleUseClosureCall(node)) return;
      if (specializeClosureCall(node)) return;
  
      AbstractValue receiver = getValue(node.receiver.definition);
  