Determine whether/how we should adopt dynamic colors.

Progress

• Introduce dynamic colors
• apply dynamic colors to cupertino components
  • action_sheet.dart
  • activity_indicator.dart
  • app.dart
  • bottom_tab_bar.dart
  • button.dart
  • date_picker.dart
  • dialog.dart
  • nav_bar.dart
  • page_scaffold.dart
  • picker.dart
  • refresh.dart
  • route.dart
  • scrollbar.dart
    - [ ] segmented_control.dart Going to make this a new widget.
  • slider.dart
  • switch.dart
  • tab_scaffold.dart -- PR opened
  • text_field.dart -- PR opened
  • text_selection.dart -- CupertinoTheme & CupertinoTextTheme dark mode updates #41859
  • text_theme.dart -- CupertinoTheme & CupertinoTextTheme dark mode updates #41859
  • theme.dart -- PR opened
  • thumb_painter.dart
• Make a new widget to succeed CupertinoSegmentedControl [iOS 13] segmented control is different #33796.
• update flutter_gallery -- PR opened Update cupertino demos in gallery #43841

Nice to have

- [ ] https://github.com/flutter/flutter/pull/41859/files#r331609336 MORE ASSERTS, e.g. if a CupertinoDynamicColor is used to paint before being resolved. Can't because that will break people using CupertinoColors.activeBlue

• Some dynamic colors should be given canonical names (such as systemBlue) and should show up nicely in devtools/logs. CupertinoDynamicColor improvements #44317
• Code-gen constants as much as possible: https://github.com/flutter/flutter/pull/41859/files#r336199469