[web] Remove /#/ from home page URL (#42598)

To make the `HashUrlStrategy` a bit cleaner (at least for the home page), let's get rid of the notorious `/#/` when the app is in the home page.

Non-home pages continue to have a hash in them. After this PR, here are some example URL changes:

- `http://domain.com/#/` => `http://domain.com/`
- `http://domain.com/#/page1` => (remains the same)

This change is backwards compatible, i.e. if you write `http://domain.com/#/` in the address bar, the app will load and open the home page correctly; and flutter will automatically change the URL to `http://domain.com/`.

Fixes flutter/flutter#127608

lib/web_ui/lib/ui_web/src/ui_web/navigation/url_strategy.dart

@@ -179,8 +179,17 @@ class HashUrlStrategy implements UrlStrategy {
     // if the empty URL is pushed it won't replace any existing fragment. So
     // when the hash path is empty, we still return the location's path and
     // query.
-    return '${_platformLocation.pathname}${_platformLocation.search}'
-        '${internalUrl.isEmpty ? '' : '#$internalUrl'}';
+    final String hash;
+    if (internalUrl.isEmpty || internalUrl == '/') {
+      // Let's not add the hash at all when the app is in the home page. That
+      // way, the URL of the home page is cleaner.
+      //
+      // See: https://github.com/flutter/flutter/issues/127608
+      hash = '';
+    } else {
+      hash = '#$internalUrl';
+    }
+    return '${_platformLocation.pathname}${_platformLocation.search}$hash';
   }
 
   @override

lib/web_ui/test/engine/history_test.dart

@@ -663,6 +663,23 @@ void testMain() {
       );
     });
 
+    test('removes /#/ from the home page', () {
+      const String internalUrl = '/';
+      final HashUrlStrategy strategy = HashUrlStrategy(location);
+
+      location.pathname = '/';
+      expect(strategy.prepareExternalUrl(internalUrl), '/');
+
+      location.pathname = '/main';
+      expect(strategy.prepareExternalUrl(internalUrl), '/main');
+
+      location.search = '?foo=bar';
+      expect(
+        strategy.prepareExternalUrl(internalUrl),
+        '/main?foo=bar',
+      );
+    });
+
     test('addPopStateListener fn unwraps DomPopStateEvent state', () {
       final HashUrlStrategy strategy = HashUrlStrategy(location);
       const String expected = 'expected value';