Escape path separator in regexps (#14122)

* Escape path separator in regexps

* Update platform_helper.dart

* Update stack_trace_test.dart

* Update stack_trace_test.dart

packages/flutter/test/foundation/platform_helper.dart

+// Copyright 2018 The Chromium 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:io' show Platform;
+
+/// Returns [Platform.pathSeparator], suitably escaped so as to be usable in a
+/// regular expression.
+String get pathSeparatorForRegExp {
+  switch (Platform.pathSeparator) {
+    case r'/':
+      return r'/';
+    case r'\':
+      return r'\\'; // because dividerRegExp gets inserted into regexps
+    default:
+      throw 'Unsupported platform.';
+  }
+}

packages/flutter/test/foundation/stack_trace_test.dart

@@ -2,21 +2,21 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-import 'dart:io';
-
 import 'package:flutter/foundation.dart';
 import 'package:test/test.dart';
 
+import 'platform_helper.dart';
+
 void main() {
   // TODO(8128): These tests and the filtering mechanism should be revisited to account for causal async stack traces.
 
-  final String divider = Platform.pathSeparator;
-  
+  final String dividerRegExp = pathSeparatorForRegExp;
+
   test('FlutterError.defaultStackFilter', () {
     final List<String> filtered = FlutterError.defaultStackFilter(StackTrace.current.toString().trimRight().split('\n')).toList();
     expect(filtered.length, greaterThanOrEqualTo(4));
     expect(filtered[0], matches(r'^#0 +main\.<anonymous closure> \(.*stack_trace_test\.dart:[0-9]+:[0-9]+\)$'));
-    expect(filtered[1], matches(r'^#1 +Declarer\.test\.<anonymous closure>.<anonymous closure> \(package:test' + divider + r'.+:[0-9]+:[0-9]+\)$'));
+    expect(filtered[1], matches(r'^#1 +Declarer\.test\.<anonymous closure>.<anonymous closure> \(package:test' + dividerRegExp + r'.+:[0-9]+:[0-9]+\)$'));
     expect(filtered[2], equals('<asynchronous suspension>'));
     expect(filtered.last, matches(r'^\(elided [1-9][0-9]+ frames from package dart:async, package dart:async-patch, and package stack_trace\)$'));
   });