msglist test: Ensure later errors get reported in full too

Otherwise, if several different test cases in this file fail due to
checks failing inside checkInvariants, then only the first one gets
reported in detail with the comparison and the stack trace.

Author: gnprice

test/model/message_list_test.dart

@@ -1,6 +1,7 @@
 import 'dart:convert';
 
 import 'package:checks/checks.dart';
+import 'package:flutter/foundation.dart';
 import 'package:http/http.dart' as http;
 import 'package:test/scaffolding.dart';
 import 'package:zulip/api/backoff.dart';
@@ -27,6 +28,24 @@ const newestResult = eg.newestGetMessagesResult;
 const olderResult = eg.olderGetMessagesResult;
 
 void main() {
+  // Arrange for errors caught within the Flutter framework to be printed
+  // unconditionally, rather than throttled as they normally are in an app.
+  //
+  // When using `testWidgets` from flutter_test, this is done automatically;
+  // compare the [FlutterError.dumpErrorToConsole] call sites,
+  // and [FlutterError.onError=] and [debugPrint=] call sites, in flutter_test.
+  //
+  // This test file is unusual in needing this manual arrangement; it's needed
+  // because these aren't widget tests, and yet do have some failures arise as
+  // exceptions that get caught by the framework: namely, when [checkInvariants]
+  // throws from within an `addListener` callback.  Those exceptions get caught
+  // by [ChangeNotifier.notifyListeners] and reported there through
+  // [FlutterError.reportError].
+  debugPrint = debugPrintSynchronously;
+  FlutterError.onError = (details) {
+    FlutterError.dumpErrorToConsole(details, forceReport: true);
+  };
+
   // These variables are the common state operated on by each test.
   // Each test case calls [prepare] to initialize them.
   late Subscription subscription;