Deprecated class Null safety

The annotation @Deprecated('migration') marks a feature as deprecated.

The annotation deprecated is a shorthand for deprecating until an unspecified "next release" without migration instructions.

A feature can be any part of an API, from a full library to a single parameter.

The intent of the @Deprecated annotation is to inform authors who are currently using the feature, that they will soon need to stop using that feature in their code, even if the feature is currently still working correctly.

Deprecation is an early warning that the deprecated feature is scheduled to be removed at a later time, a time possibly specified in message. A deprecated feature should no longer be used, code using it will break at some point in the future. If existing code is using the feature, that code should be rewritten to no longer use the deprecated feature.

A deprecated feature should document how the same effect can be achieved in message, so the programmer knows how to rewrite the code.

The @Deprecated annotation applies to libraries, top-level declarations (variables, getters, setters, functions, classes, mixins, extension and typedefs), class-level declarations (variables, getters, setters, methods, operators or constructors, whether static or not), named optional parameters and trailing optional positional parameters.

Deprecation applies transitively to parts of a deprecated feature:

• If a library is deprecated, so is every member of it.
• If a class is deprecated, so is every member of it.
• If a variable is deprecated, so are its implicit getter and setter.

If a feature is deprecated in a superclass, it is not automatically deprecated in a subclass as well. It is reasonable to remove a member from a superclass and retain it in a subclass, so it needs to be possible to deprecate the member only in the superclass.

A tool that processes Dart source code may report when:

• the code imports a deprecated library.
• the code exports a deprecated library, or any deprecated member of a non-deprecated library.
• the code refers statically to a deprecated declaration.
• the code uses a member of an object with a statically known type, where the member is deprecated on the interface of the static type.
• the code calls a method with an argument where the corresponding optional parameter is deprecated on the object's static type.

If the deprecated use is inside a library, class or method which is itself deprecated, the tool should not bother the user about it. A deprecated feature is expected to use other deprecated features.