Add M3 date range picker tests and fix header background theme color (#127662)

packages/flutter/lib/src/material/date_picker.dart

@@ -13,7 +13,6 @@ import 'back_button.dart';
 import 'button_style.dart';
 import 'calendar_date_picker.dart';
 import 'color_scheme.dart';
-import 'colors.dart';
 import 'date.dart';
 import 'date_picker_theme.dart';
 import 'debug.dart';
@@ -1588,6 +1587,7 @@ class _CalendarRangePickerDialog extends StatelessWidget {
     final DatePickerThemeData themeData = DatePickerTheme.of(context);
     final DatePickerThemeData defaults = DatePickerTheme.defaults(context);
     final Color? dialogBackground = themeData.rangePickerBackgroundColor ?? defaults.rangePickerBackgroundColor;
+    final Color? headerBackground = themeData.rangePickerHeaderBackgroundColor ?? defaults.rangePickerHeaderBackgroundColor;
     final Color? headerForeground = themeData.rangePickerHeaderForegroundColor ?? defaults.rangePickerHeaderForegroundColor;
     final Color? headerDisabledForeground = headerForeground?.withOpacity(0.38);
     final TextStyle? headlineStyle = themeData.rangePickerHeaderHeadlineStyle ?? defaults.rangePickerHeaderHeadlineStyle;
@@ -1616,7 +1616,7 @@ class _CalendarRangePickerDialog extends StatelessWidget {
           actionsIconTheme: iconTheme,
           elevation: useMaterial3 ? 0 : null,
           scrolledUnderElevation: useMaterial3 ? 0 : null,
-          backgroundColor: useMaterial3 ? Colors.transparent : null,
+          backgroundColor: useMaterial3 ? headerBackground : null,
           leading: CloseButton(
             onPressed: onCancel,
           ),

packages/flutter/test/material/date_picker_theme_test.dart

@@ -431,6 +431,9 @@ void main() {
     expect(material.surfaceTintColor, datePickerTheme.rangePickerSurfaceTintColor);
     expect(material.shape, datePickerTheme.rangePickerShape);
 
+    final AppBar appBar = tester.widget<AppBar>(find.byType(AppBar));
+    expect(appBar.backgroundColor, datePickerTheme.rangePickerHeaderBackgroundColor);
+
     final Text selectRange = tester.widget<Text>(find.text('Select range'));
     expect(selectRange.style?.color, datePickerTheme.rangePickerHeaderForegroundColor);
     expect(selectRange.style?.fontSize, datePickerTheme.rangePickerHeaderHelpStyle?.fontSize);