 href="dart-io/FileLock/exclusive-constant.html">FileLock.exclusive</a> or <a href="dart-io/FileLock/shared-constant.html">FileLock.shared</a>, an error is
signaled if the lock cannot be obtained. If <code>mode</code> is
<a href="dart-io/FileLock/blockingExclusive-constant.html">FileLock.blockingExclusive</a> or <a href="dart-io/FileLock/blockingShared-constant.html">FileLock.blockingShared</a>, the
returned <a href="dart-async/Future-class.html">Future</a> is resolved only when the lock has been obtained.<