'migrate_design_widgets' fix does not migrate exports

[loic-sharma]

Repro steps

Create a Flutter app that contains:

export 'package:flutter/material.dart';

Using the latest Flutter main channel, run:

dart fix --apply --code=migrate_design_widgets 

I would expect the following migration:

- export 'package:flutter/material.dart';
+ export 'package:material_ui/material_ui.dart';

Affected apps

• Wonderous app: https://github.com/gskinnerTeam/flutter-wonderous-app/blob/747b945a7e5239356bf2664261aa2f3b020b8898/lib/common_libs.dart#L9
• Community projects: https://github.com/search?type=code&q=%22export+%27package%3Aflutter%2Fmaterial.dart%27%3B%22

[keertip]

/cc @keertip @bwilkerson

[bwilkerson]

Given the number of community projects that are exporting these widgets, we should consider cherry-picking the CL for this issue.