Adds the arrowColor option to UserAccountsDrawerHeader (#38608) (#38636)

Changes a hard-coded value to a controllable parameter.

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

@@ -72,12 +72,14 @@ class _AccountDetails extends StatefulWidget {
     @required this.accountEmail,
     this.onTap,
     this.isOpen,
+    this.arrowColor,
   }) : super(key: key);
 
   final Widget accountName;
   final Widget accountEmail;
   final VoidCallback onTap;
   final bool isOpen;
+  final Color arrowColor;
 
   @override
   _AccountDetailsState createState() => _AccountDetailsState();
@@ -179,7 +181,7 @@ class _AccountDetailsState extends State<_AccountDetails> with SingleTickerProvi
                 angle: _animation.value * math.pi,
                 child: Icon(
                   Icons.arrow_drop_down,
-                  color: Colors.white,
+                  color: widget.arrowColor,
                   semanticLabel: widget.isOpen
                     ? localizations.hideAccountsLabel
                     : localizations.showAccountsLabel,
@@ -314,6 +316,7 @@ class UserAccountsDrawerHeader extends StatefulWidget {
     @required this.accountName,
     @required this.accountEmail,
     this.onDetailsPressed,
+    this.arrowColor = Colors.white,
   }) : super(key: key);
 
   /// The header's background. If decoration is null then a [BoxDecoration]
@@ -344,6 +347,9 @@ class UserAccountsDrawerHeader extends StatefulWidget {
   /// [accountName] and [accountEmail] is tapped.
   final VoidCallback onDetailsPressed;
 
+  /// The [Color] of the arrow icon.
+  final Color arrowColor;
+
   @override
   _UserAccountsDrawerHeaderState createState() => _UserAccountsDrawerHeaderState();
 }
@@ -390,6 +396,7 @@ class _UserAccountsDrawerHeaderState extends State<UserAccountsDrawerHeader> {
                 accountEmail: widget.accountEmail,
                 isOpen: _isOpen,
                 onTap: widget.onDetailsPressed == null ? null : _handleDetailsPressed,
+                arrowColor: widget.arrowColor,
               ),
             ],
           ),

packages/flutter/test/material/user_accounts_drawer_header_test.dart

@@ -216,6 +216,37 @@ void main() {
     expect(transformWidget.transform.getRotation()[4], 1.0);
   });
 
+  testWidgets('UserAccountsDrawerHeader icon color changes', (WidgetTester tester) async {
+    await tester.pumpWidget(MaterialApp(
+      home: Material(
+        child: UserAccountsDrawerHeader(
+          onDetailsPressed: () {},
+          accountName: const Text('name'),
+          accountEmail: const Text('email'),
+        ),
+      ),
+    ));
+
+    Icon iconWidget = tester.firstWidget(find.byType(Icon));
+    // Default icon color is white.
+    expect(iconWidget.color, Colors.white);
+
+    const Color arrowColor = Colors.red;
+    await tester.pumpWidget(MaterialApp(
+      home: Material(
+        child: UserAccountsDrawerHeader(
+          onDetailsPressed: () { },
+          accountName: const Text('name'),
+          accountEmail: const Text('email'),
+          arrowColor: arrowColor,
+        ),
+      ),
+    ));
+
+    iconWidget = tester.firstWidget(find.byType(Icon));
+    expect(iconWidget.color, arrowColor);
+  });
+
   testWidgets('UserAccountsDrawerHeader null parameters LTR', (WidgetTester tester) async {
     Widget buildFrame({
       Widget currentAccountPicture,