remove pending timers list code out of assert message (#57287)

* move pending timers list code out of assert

* style change
Co-authored-by: Emmanuel Garcia <egarciad@google.com>

* add unit tests

* formatting

* pending timer test

* pending timer assertion test

* add setup and teardown
Co-authored-by: Emmanuel Garcia <egarciad@google.com>

packages/flutter_test/lib/src/binding.dart

@@ -1116,23 +1116,20 @@ class AutomatedTestWidgetsFlutterBinding extends TestWidgetsFlutterBinding {
   void _verifyInvariants() {
     super._verifyInvariants();
 
-    assert(() {
-      if (   _currentFakeAsync.periodicTimerCount == 0
-          && _currentFakeAsync.nonPeriodicTimerCount == 0) {
-        return true;
-      }
-
-      debugPrint('Pending timers:');
-      for (final FakeTimer timer in _currentFakeAsync.pendingTimers) {
-        debugPrint(
-          'Timer (duration: ${timer.duration}, '
-          'periodic: ${timer.isPeriodic}), created:');
-        debugPrintStack(stackTrace: timer.creationStackTrace);
-        debugPrint('');
-      }
-      return false;
-    }(), 'A Timer is still pending even after the widget tree was disposed.');
-
+    bool timersPending = false;
+    if (_currentFakeAsync.periodicTimerCount != 0 ||
+        _currentFakeAsync.nonPeriodicTimerCount != 0) {
+        debugPrint('Pending timers:');
+        for (final FakeTimer timer in _currentFakeAsync.pendingTimers) {
+          debugPrint(
+            'Timer (duration: ${timer.duration}, '
+            'periodic: ${timer.isPeriodic}), created:');
+          debugPrintStack(stackTrace: timer.creationStackTrace);
+          debugPrint('');
+        }
+        timersPending = true;
+    }
+    assert(!timersPending, 'A Timer is still pending even after the widget tree was disposed.');
     assert(_currentFakeAsync.microtaskCount == 0); // Shouldn't be possible.
   }
 

packages/flutter_test/test/widget_tester_test.dart

@@ -775,6 +775,33 @@ void main() {
       expect(desktop.values.union(mobile.values), equals(all.values));
     });
   });
+
+  group('Pending timer', () {
+    TestExceptionReporter currentExceptionReporter;
+    setUp(() {
+      currentExceptionReporter = reportTestException;
+    });
+
+    tearDown(() {
+      reportTestException = currentExceptionReporter;
+    });
+
+    test('Throws assertion message without code', () async {
+      FlutterErrorDetails flutterErrorDetails;
+      reportTestException = (FlutterErrorDetails details, String testDescription) {
+        flutterErrorDetails = details;
+      };
+
+      final TestWidgetsFlutterBinding binding = TestWidgetsFlutterBinding.ensureInitialized() as TestWidgetsFlutterBinding;
+      await binding.runTest(() async {
+        final Timer timer = Timer(const Duration(seconds: 1), () {});
+        expect(timer.isActive, true);
+      }, () {});
+
+      expect(flutterErrorDetails?.exception, isA<AssertionError>());
+      expect(flutterErrorDetails?.exception?.message, 'A Timer is still pending even after the widget tree was disposed.');
+    });
+  });
 }
 
 class FakeMatcher extends AsyncMatcher {