msglist: Make DM recipient header icon (ZulipIcons.user) match text color

As it does in the Figma.

(Before, the color was the Material constant kDefaultIconDarkColor,
which is Color(0xDD000000).)

Author: chrisbobbe

lib/widgets/message_list.dart

@@ -765,9 +765,12 @@ class DmRecipientHeader extends StatelessWidget {
           child: Row(
             crossAxisAlignment: CrossAxisAlignment.center,
             children: [
-              const Padding(
-                padding: EdgeInsets.symmetric(horizontal: 6),
-                child: Icon(size: 16, ZulipIcons.user)),
+              Padding(
+                padding: const EdgeInsets.symmetric(horizontal: 6),
+                child: Icon(
+                  color: _kRecipientHeaderTextColor,
+                  size: 16,
+                  ZulipIcons.user)),
               Expanded(
                 child: Text(title,
                   style: recipientHeaderTextStyle(context),

test/flutter_checks.dart

@@ -66,6 +66,7 @@ extension TextFieldChecks on Subject<TextField> {
 
 extension TextStyleChecks on Subject<TextStyle> {
   Subject<bool> get inherit => has((t) => t.inherit, 'inherit');
+  Subject<Color?> get color => has((t) => t.color, 'color');
   Subject<double?> get fontSize => has((t) => t.fontSize, 'fontSize');
   Subject<FontWeight?> get fontWeight => has((t) => t.fontWeight, 'fontWeight');
   Subject<double?> get letterSpacing => has((t) => t.letterSpacing, 'letterSpacing');

test/widgets/message_list_test.dart

@@ -1,7 +1,9 @@
 import 'dart:convert';
 
 import 'package:checks/checks.dart';
+import 'package:collection/collection.dart';
 import 'package:flutter/material.dart';
+import 'package:flutter/rendering.dart';
 import 'package:flutter_gen/gen_l10n/zulip_localizations.dart';
 import 'package:flutter_test/flutter_test.dart';
 import 'package:http/http.dart' as http;
@@ -399,6 +401,19 @@ void main() {
         tester.widget(find.text(zulipLocalizations.messageListGroupYouAndOthers(
           "${zulipLocalizations.unknownUserName}, ${eg.thirdUser.fullName}")));
       });
+
+      testWidgets('icon color matches text color', (tester) async {
+        final zulipLocalizations = GlobalLocalizations.zulipLocalizations;
+        await setupMessageListPage(tester, messages: [
+          eg.dmMessage(from: eg.otherUser, to: [eg.selfUser]),
+        ]);
+        await tester.pump();
+        final textSpan = tester.renderObject<RenderParagraph>(find.text(
+          zulipLocalizations.messageListGroupYouAndOthers(
+            zulipLocalizations.unknownUserName))).text;
+        final icon = tester.widget<Icon>(find.byIcon(ZulipIcons.user));
+        check(textSpan).style.isNotNull().color.equals(icon.color);
+      });
     });
 
     testWidgets('show dates', (tester) async {