[web] Don't allow empty initial route (#17936)

lib/web_ui/lib/src/engine/browser_location.dart

@@ -73,10 +73,13 @@ class HashLocationStrategy extends LocationStrategy {
     // and if it is empty then it will stay empty
     String path = _platformLocation.hash ?? '';
     assert(path.isEmpty || path.startsWith('#'));
-    // Dart will complain if a call to substring is
-    // executed with a position value that exceeds the
-    // length of string.
-    return path.isEmpty ? path : path.substring(1);
+
+    // We don't want to return an empty string as a path. Instead we default to "/".
+    if (path.isEmpty || path == '#') {
+      return '/';
+    }
+    // At this point, we know [path] starts with "#" and isn't empty.
+    return path.substring(1);
   }
 
   @override

lib/web_ui/test/engine/history_test.dart

@@ -7,6 +7,7 @@
 // TODO(nurhan): https://github.com/flutter/flutter/issues/51169
 
 import 'dart:async';
+import 'dart:html' as html;
 import 'dart:typed_data';
 
 import 'package:test/test.dart';
@@ -28,7 +29,7 @@ const MethodCodec codec = JSONMethodCodec();
 void emptyCallback(ByteData date) {}
 
 void main() {
-  group('BrowserHistory', () {
+  group('$BrowserHistory', () {
     final PlatformMessagesSpy spy = PlatformMessagesSpy();
 
     setUp(() {
@@ -229,6 +230,41 @@ void main() {
         // TODO(nurhan): https://github.com/flutter/flutter/issues/50836
         skip: browserEngine == BrowserEngine.edge);
   });
+
+  group('$HashLocationStrategy', () {
+    TestPlatformLocation location;
+
+    setUp(() {
+      location = TestPlatformLocation();
+    });
+
+    tearDown(() {
+      location = null;
+    });
+
+    test('leading slash is optional', () {
+      final HashLocationStrategy strategy = HashLocationStrategy(location);
+
+      location.hash = '#/';
+      expect(strategy.path, '/');
+
+      location.hash = '#/foo';
+      expect(strategy.path, '/foo');
+
+      location.hash = '#foo';
+      expect(strategy.path, 'foo');
+    });
+
+    test('path should not be empty', () {
+      final HashLocationStrategy strategy = HashLocationStrategy(location);
+
+      location.hash = '';
+      expect(strategy.path, '/');
+
+      location.hash = '#';
+      expect(strategy.path, '/');
+    });
+  });
 }
 
 void pushRoute(String routeName) {
@@ -276,3 +312,38 @@ Future<void> systemNavigatorPop() {
   );
   return completer.future;
 }
+
+/// A mock implementation of [PlatformLocation] that doesn't access the browser.
+class TestPlatformLocation extends PlatformLocation {
+  String pathname;
+  String search;
+  String hash;
+
+  void onPopState(html.EventListener fn) {
+    throw UnimplementedError();
+  }
+
+  void offPopState(html.EventListener fn) {
+    throw UnimplementedError();
+  }
+
+  void onHashChange(html.EventListener fn) {
+    throw UnimplementedError();
+  }
+
+  void offHashChange(html.EventListener fn) {
+    throw UnimplementedError();
+  }
+
+  void pushState(dynamic state, String title, String url) {
+    throw UnimplementedError();
+  }
+
+  void replaceState(dynamic state, String title, String url) {
+    throw UnimplementedError();
+  }
+
+  void back() {
+    throw UnimplementedError();
+  }
+}