prefer_for_elements_to_map_fromIterable

When building maps from iterables, it is preferable to use 'for' elements.

Using 'for' elements brings several benefits including:

• Performance
• Flexibility
• Readability
• Improved type inference
• Improved interaction with null safety

BAD:

Map<String, WidgetBuilder>.fromIterable(
  kAllGalleryDemos,
  key: (demo) => '${demo.routeName}',
  value: (demo) => demo.buildRoute,
);

GOOD:

return {
  for (var demo in kAllGalleryDemos)
    '${demo.routeName}': demo.buildRoute,
};

GOOD:

// Map<int, Student> is not required, type is inferred automatically.
final pizzaRecipients = {
  ...studentLeaders,
  for (var student in classG)
    if (student.isPassing) student.id: student,
};