For example, Dart web apps generally use the library, which they can import like this:

The only required argument to import is a URI specifying thelibrary.For built-in libraries, the URI has the special dart: scheme.For other libraries, you can use a file system path or the package:scheme. The package: scheme specifies libraries provided by a packagemanager such as the pub tool. For example:

import 'package:test/test.dart';

Note:URI stands for uniform resource identifier. URLs (uniform resource locators) are a common kind of URI.

Specifying a library prefix

Importing only part of a library

If you want to use only part of a library, you can selectively importthe library. For example:

// Import only foo.
// Import all names EXCEPT foo.
import 'package:lib2/lib2.dart' hide foo;

Lazily loading a library

Deferred loading (also called lazy loading)allows a web app to load a library on demand,if and when the library is needed.Here are some cases when you might use deferred loading:

• To reduce a web app’s initial startup time.
• To perform A/B testing—trying outalternative implementations of an algorithm, for example.

To lazily load a library, you must firstimport it using deferred as.

When you need the library, invokeloadLibrary() using the library’s identifier.

Future greet() async {
  await hello.loadLibrary();
  hello.printGreeting();

In the preceding code,the await keyword pauses execution until the library is loaded.For more information about async and await,see .

Keep in mind the following when you use deferred loading:

• A deferred library’s constants aren’t constants in the importing file.Remember, these constants don’t exist until the deferred library is loaded.
• Dart implicitly inserts loadLibrary() into the namespace that you defineusing deferred as namespace.The loadLibrary() function returns a Future.