Dart Libraries #1
In this lesson, you will be introduced to the Dart libraries.
Introduction
This lesson is an introduction to libraries in Dart/Flutter. A library is a reusable module for frequently used programs/code. It helps to write a modular codebase. In Dart, each app is a library.
The lesson about libraries is divided into two lessons.
Dart Libraries #1: The first lesson covers how to use Dart libraries including the following:
 Using Prefix for library
 Importing specific APIs
 Defer/delay loading library
Dart Libraries #2: The second lesson instructs on understanding the following directives:
 The
part
directive  The
library
directive  The
export
directive
Setup
Let’s create two libraries lib1
and lib2
for demonstration.
lib1
This library has APIs to calculate the sum and difference of two integers.

Addition API: The
int addition(int a, int b)
calculates the sum of two given integers and returns an integer. 
Subtraction API: The
int subtraction(int a, int b)
calculates the difference of two given integers and returns an integer. 
Internal Method
int _add(int a, int b)
: It is an internal function/method that performs the real addition. This method is used by the addition API. Identifiers with the underscore (_) prefix are only available inside the library.
File ‘lib1.dart’:
int addition(int a, int b) => _add(a, b);
int subtraction(int a, int b) => a  b;
int _add(int a, int b) => a + b;
lib2
This library has APIs to check if the given numbers are even or odd.
 Even number API (
bool isNumberOdd(int num)
): This API takes a number and returns true if it is even or vice versa.  Odd number API (
bool isNumberOdd(int num)
): This API takes a number and returns true if it is odd or vice versa.
File ‘lib2.dart’:
bool isNumberOdd(int num) => num.isOdd;
bool isNumberEven(int num) => num.isEven;
//Added to demonstrate handle conflicting name in two different libraries
int addition(int a, int b) => a + b + 2;
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