The property sector in the Chinese economy has always been something of a puzzle. At its peak, it accounted for a quarter of the nation’s economic output, broadly measured. And it sees people in Beijing and Shanghai paying house prices similar to those in San Francisco and New York, despite having just a quarter the income of American buyers. Now many believe that we are about to see a violent contraction of the property market in China. The government wants to intervene to curb speculation, and rein in what it…