Fix issue with stack traces getting mangled (#59900)

packages/flutter/lib/src/foundation/assertions.dart

@@ -389,6 +389,28 @@ class FlutterErrorDetails with Diagnosticable {
     this.silent = false,
   });
 
+  /// Creates a copy of the error details but with the given fields replaced
+  /// with new values.
+  FlutterErrorDetails copyWith({
+    DiagnosticsNode context,
+    dynamic exception,
+    InformationCollector informationCollector,
+    String library,
+    bool silent,
+    StackTrace stack,
+    IterableFilter<String> stackFilter,
+  }) {
+    return FlutterErrorDetails(
+      context: context ?? this.context,
+      exception: exception ?? this.exception,
+      informationCollector: informationCollector ?? this.informationCollector,
+      library: library ?? this.library,
+      silent: silent ?? this.silent,
+      stack: stack ?? this.stack,
+      stackFilter: stackFilter ?? this.stackFilter,
+    );
+  }
+
   /// Transformers to transform [DiagnosticsNode] in [DiagnosticPropertiesBuilder]
   /// into a more descriptive form.
   ///

packages/flutter_test/lib/src/binding.dart

@@ -579,6 +579,10 @@ abstract class TestWidgetsFlutterBinding extends BindingBase
       // our main future completing.
       assert(Zone.current == _parentZone);
       if (_pendingExceptionDetails != null) {
+        assert(
+          _unmangle(_pendingExceptionDetails.stack) == _pendingExceptionDetails.stack,
+          'The test binding presented an unmangled stack trace to the framework.',
+        );
         debugPrint = debugPrintOverride; // just in case the test overrides it -- otherwise we won't see the error!
         reportTestException(_pendingExceptionDetails, testDescription);
         _pendingExceptionDetails = null;
@@ -611,6 +615,7 @@ abstract class TestWidgetsFlutterBinding extends BindingBase
     _oldExceptionHandler = FlutterError.onError;
     int _exceptionCount = 0; // number of un-taken exceptions
     FlutterError.onError = (FlutterErrorDetails details) {
+      details = details.copyWith(stack: _unmangle(details.stack));
       if (_pendingExceptionDetails != null) {
         debugPrint = debugPrintOverride; // just in case the test overrides it -- otherwise we won't see the errors!
         if (_exceptionCount == 0) {

packages/flutter_test/test/bindings_async_gap_test.dart

@@ -0,0 +1,52 @@
+// Copyright 2014 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'dart:async';
+
+import 'package:flutter/foundation.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:stack_trace/stack_trace.dart' as stack_trace;
+
+Future<void> main() async {
+  test('demangles stacks', () async {
+    // Test that the tester bindings unmangle stacks that come in as
+    // package:stack_trace types.
+    // Uses runTest directly so that the test does not get hung up waiting for
+    // the error reporter to be reset to the original one.
+
+    final Completer<FlutterErrorDetails> errorCompleter = Completer<FlutterErrorDetails>();
+    final TestExceptionReporter oldReporter = reportTestException;
+    reportTestException = (FlutterErrorDetails details, String testDescription) {
+      errorCompleter.complete(details);
+      reportTestException = oldReporter;
+    };
+
+    final AutomatedTestWidgetsFlutterBinding binding = AutomatedTestWidgetsFlutterBinding();
+    await binding.runTest(() async {
+      final Completer<String> completer = Completer<String>();
+
+      completer.future.then(
+        (String value) {},
+        onError: (dynamic error, StackTrace stack) {
+          assert(stack is stack_trace.Chain);
+          FlutterError.reportError(FlutterErrorDetails(
+            exception: error,
+            stack: stack,
+          ));
+        }
+      );
+
+      completer.completeError(const CustomException());
+    }, null);
+
+    final FlutterErrorDetails details = await errorCompleter.future;
+    expect(details, isNotNull);
+    expect(details.exception, isA<CustomException>());
+    reportTestException = oldReporter;
+  });
+}
+
+class CustomException implements Exception {
+  const CustomException();
+}