The classic American plastic yo-yo is becoming big business in China. With savvy promotion by Chinese toy companies, the market has grown from barely a blip six years ago to one of the largest, with one U.S. firm saying China is now the world’s top selling yo-yo market. As with cars, where China rose quickly to become the biggest market worldwide, the new popularity of yo-yos is getting noticed by foreign companies. The largest yo-yo maker in the United States, Duncan Toys Co., recently launched its first products for China and the company sees significant opportunity. Duncan a division of Baraboo, Wis.-based plastics firm Flambeau Inc. set up an office in China in 2009. It hired former Chinese national yo-yo champion Luo Jianbin for market development and last year received a China sales license.