Create a Dart class named Book with the following specifications:
Properties: title (String), author (String), and numberOfPages (int). A constructor that initializes all the properties. A method named bookDetails that prints a string containing all the properties.
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Create a class named FictionBook that inherits from the Book class created in Task 1. Add a new property named genre (String) and override the bookDetails method to include the genre in the output.
Define a mixin named Readable that provides a method read. The read method should print out a message indicating that the book is being read.
Write a Dart function named calculateArea that takes two named parameters length and width (both of type double) and an optional parameter shape of type String with a default value of "rectangle". The function should return the area as a double. If shape is "circle", use length as the radius to calculate the area of the circle (use 3.14 as the value of pi).
Create a Dart class named User with the following properties:
name (String): Required parameter. age (int?): Nullable parameter. email (String?): Nullable parameter. The class should have a constructor that initializes all the properties.
Additionally, create a method named printUserDetails that prints the user details in the following format:
Name: [user name] Age: [user age] Email: [user email]
If the age or email is null, it should print "N/A" instead of the actual value.