For decades, an annual gathering of oncologists has featured drug trials that were run mainly at American and European hospitals. But at this year’s meeting, which is being held in Chicago this weekend, the signs are everywhere of China’s ascendance as a powerhouse in drug development — and of the threat that many believe it poses to American biotechnology. The clearest sign: In what appears to be a first, one of the conference’s five coveted headliners is a presentation of a clinical trial conducted only in China. That milestone at…