Regression testing is an important part of the development process but suffers from the presence of flaky tests. Flaky tests nondeterministically pass or fail when run on the same code, misleading developers about the correctness of their changes. A common type of flaky tests are async flaky tests that flakily fail due to timing-related issues such as asynchronous waits that do not return in time or different thread interleavings during execution. Developers commonly try to repair async flaky tests by inserting or increasing some wait time, but such repairs are unreliable. We propose FlakeSync, a technique for automatically repairing async flaky tests by introducing synchronization for a specific test execution. FlakeSync works by identifying a critical point, representing some key part of code that must be executed early w.r.t. other concurrently executing code, and a barrier point, representing the part of code that should wait until the critical point has been executed. FlakeSync can modify code to check when the critical point is executed and have the barrier point keep waiting until the critical point has been executed, essentially synchronizing these two parts of code for the specific test execution. Our evaluation of FlakeSync on known flaky tests from prior work shows that FlakeSync can automatically repair 83.75% of async flaky tests, and the resulting changes add a median overhead of only 1.00X the original test runtime. We submitted 10 pull requests with our changes to developers, with 3 already accepted and none rejected.