Migration tool should convert `@required` to new `required` keyword

[stereotype441]

Consider the following code:

import 'package:meta/meta.dart';
void f({@required String s}) {}

The migration tool currently leaves this code untouched. It should be changed so that it changes @required to the new required keyword.

[stereotype441]

As of e0f97cd, this works.

[stereotype441]

This is not working everywhere. See, for example, in flutter's lib/web_ui/lib/src/engine/text/paragraph.dart:37.