anchor.diff:        var bodyBlock = currentBlock = emptyBlock();
   
         breakTargets.add(identifier(breakName));
  -      breakLabels.add(breakName);
  +      breakLabels.add(null);
   
         continueTargets.add(identifier(continueName));
   
  
positive.code: 
    ast.Block currentBlock;
    List<ast.Expression> breakTargets;
    List<ast.Expression> breakLabels;
  
    List<ast.Expression> continueTargets;
    List<ast.Expression> continueLabels;
  
    visit(ast.AstNode node) => node.accept(this);
  
  
negative.code:     var target;
  
      if (node.label != null) {
        for (var i = breakLabels.length - 1; i >= 0; i--) {
          if(breakLabels[i] == node.label.name) {
            target = breakTargets[i];
            break;
          }
        }
      } else {
        target = breakTargets.last;
      }
  