In Dart, asynchronous operations are handled using asynchronous programming techniques, which allow you to perform operations without blocking the execution of your code. Asynchronous programming is particularly useful when dealing with time-consuming tasks such as network requests, file operations, or interactions with databases. Dart provides several mechanisms to handle asynchronous operations:

1. Future and async/await : Dart utilizes the `Future` class, which represents a value that may not be available immediately. The `async` and `await` keywords are used together to work with asynchronous operations in a synchronous manner, making the code more readable and easier to follow.

   * `async`: The `async` keyword is used to mark a function as asynchronous. It allows you to use the `await` keyword within the function body to wait for the completion of asynchronous operations.

   * `await`: The `await` keyword is used to pause the execution of an `async` function until a `Future` completes. It allows you to extract the result of the `Future` once it's available.

   Here's an example that demonstrates the usage of `async` and `await`:

   Future<void> fetchData() async {
     // Simulating an asynchronous operation
     await Future.delayed(Duration(seconds: 2));
     print('Data fetched!');
   }

   void main() {
     print('Start');
     fetchData();
     print('End');
   }​

   Output :

   Start
   End
   Data fetched!​

   In the example above, the `fetchData` function is marked as `async`, and the `await` keyword is used to pause the execution until the `Future.delayed` operation completes. The delay simulates an asynchronous operation, and once the delay is over, the code resumes execution, and "Data fetched!" is printed.

2. Callbacks and Futures : Dart also provides the option to handle asynchronous operations using callbacks and `Future` objects directly. This approach involves registering callbacks to be executed when a `Future` completes.

   void fetchData() {
     Future.delayed(Duration(seconds: 2)).then((_) {
       print('Data fetched!');
     });
   }

   void main() {
     print('Start');
     fetchData();
     print('End');
   }​

   Output :

   Start
   End
   Data fetched!​

   In this example, the `then` method is used to register a callback to be executed when the `Future.delayed` operation completes. Once the delay is over, the callback is invoked, and "Data fetched!" is printed.

3. Streams : Dart provides a `Stream` class to handle continuous asynchronous data streams, such as real-time data feeds or event streams. Streams are a sequence of asynchronous events that can be listened to and processed.

   Stream<int> countDown(int from) async* {
     for (int i = from; i >= 0; i--) {
       await Future.delayed(Duration(seconds: 1));
       yield i;
     }
   }

   void main() {
     print('Start');
     countDown(5).listen((value) {
       print(value);
     });
     print('End');
   }​

   Output :

   Start
   End
   5
   4
   3
   2
   1
   0​

   In this example, the `countDown` function returns a `Stream` that emits integers from a given number down to zero. The `listen` method is used to subscribe to the stream and receive the emitted values. As the countdown progresses, the callback inside `listen` is invoked for each emitted