In Dart, the "is" and "as" keywords are used for type checks and type casts, respectively.

1.  "is" keyword : The "is" keyword is used for type checks to determine if an object is of a specific type. It returns a boolean value, true or false, based on the object's type. It is commonly used in conditional statements or to perform type-specific operations.

   Example :

   var obj = 'Hello';
   if (obj is String) {
     print('The object is a String.');
   } else {
     print('The object is not a String.');
   }​

   In the above example, the "is" keyword is used to check if the variable `obj` is of type String. If it is, the message "The object is a String." will be printed; otherwise, the message "The object is not a String." will be printed.
2. "as" keyword : The "as" keyword is used for type casts, allowing you to explicitly cast an object to a specific type. It performs a type check at runtime and casts the object to the desired type if the check is successful. If the object cannot be cast to the specified type, a runtime exception may occur.

   Example :

   var obj = 'Hello';
   var str = obj as String;
   print(str.toUpperCase());​

   In the above example, the "as" keyword is used to cast the variable `obj` to the String type and assign it to the variable `str`. Since `obj` is already a String, the cast is successful, and the `toUpperCase()` method can be called on `str`.

   It's important to note that if the object is not of the specified type, a runtime exception, such as a `TypeError`, may occur. Therefore, it is recommended to use the "is" keyword to perform a type check before using the "as" keyword for type casting to ensure safety.

Both "is" and "as" keywords are useful when dealing with polymorphism, conditional branching based on object types, and when you need to perform specific operations based on an object's type in Dart.