title: Getting error DiagnosticableMixin after Update flutter SDK 2.0
body: <p>After Update flutter SDK 2.0 i am getting below error.</p>
  <p>../../SDK/flutter/.pub-cache/hosted/pub.dartlang.org/flutter_cupertino_date_picker-1.0.26+2/lib/src/date_picker_theme.dart:23:32: Error: Type 'DiagnosticableMixin' not found.
  class DateTimePickerTheme with DiagnosticableMixin {
  ^^^^^^^^^^^^^^^^^^^
  ../../SDK/flutter/.pub-cache/hosted/pub.dartlang.org/flutter_cupertino_date_picker-1.0.26+2/lib/src/date_picker_theme.dart:23:7: Error: The type 'DiagnosticableMixin' can't be mixed in.
  class DateTimePickerTheme with DiagnosticableMixin {
  ^
  ../../SDK/flutter/.pub-cache/hosted/pub.dartlang.org/flutter_cupertino_date_picker-1.0.26+2/lib/src/date_picker.dart:103:34: Error: No named parameter with the name 'shadowThemeOnly'.
  theme: Theme.of(context, shadowThemeOnly: true),
  ^^^^^^^^^^^^^^^
  ../../SDK/flutter/packages/flutter/lib/src/material/theme.dart:107:20: Context: Found this candidate, but the arguments don't match.
  static ThemeData of(BuildContext context) {</p>