Minimize cross imports in examples #190305

The examples, located in the package root and in packages/flutter, should avoid cross imports.

• If an example is unrelated to Material and Cupertino, consider refactoring it to only use Widgets.
• If an example is realted to Material or Cupertino, consider moving it to material_ui or cupertino_ui.

See knownExamplesCrossImports, which contains a list of all example files with cross imports. The examples in api/lib/material and api/lib/cupertino do not need to be migrated because they have already been moved to material_ui or cupertino_ui.

As discussed in #190237 (comment), there's a small amount of concern for these imports of material_ui and cupertino_ui causing complicated migrations.

Part of #177028