[web] Assert route names start with / if using PathUrlStrategy (#127986)

To avoid confusion and unpredictable errors (e.g. https://github.com/flutter/flutter/issues/127462), I'm adding this assert to disallow this pattern.

Fixes https://github.com/flutter/flutter/issues/127462

packages/flutter_web_plugins/lib/src/navigation/url_strategy.dart

@@ -63,9 +63,15 @@ class PathUrlStrategy extends ui_web.HashUrlStrategy {
 
   @override
   String prepareExternalUrl(String internalUrl) {
-    if (internalUrl.isNotEmpty && !internalUrl.startsWith('/')) {
-      internalUrl = '/$internalUrl';
+    if (internalUrl.isEmpty) {
+      internalUrl = '/';
     }
+    assert(
+      internalUrl.startsWith('/'),
+      "When using PathUrlStrategy, all route names must start with '/' because "
+      "the browser's pathname always starts with '/'. "
+      "Found route name: '$internalUrl'",
+    );
     return '$_basePath$internalUrl';
   }
 }

packages/flutter_web_plugins/test/navigation/url_strategy_test.dart

@@ -16,17 +16,16 @@ void main() {
 
     setUp(() {
       location = TestPlatformLocation();
+      location.baseHref = '/';
     });
 
     test('allows null state', () {
-      location.baseHref = '/';
       final PathUrlStrategy strategy = PathUrlStrategy(location);
       expect(() => strategy.pushState(null, '', '/'), returnsNormally);
       expect(() => strategy.replaceState(null, '', '/'), returnsNormally);
     });
 
     test('validates base href', () {
-      location.baseHref = '/';
       expect(
         () => PathUrlStrategy(location),
         returnsNormally,
@@ -58,7 +57,6 @@ void main() {
     });
 
     test('leading slash is always prepended', () {
-      location.baseHref = '/';
       final PathUrlStrategy strategy = PathUrlStrategy(location);
 
       location.pathname = '';
@@ -95,13 +93,35 @@ void main() {
       expect(strategy.getPath(), '/bar?q=1&t=r');
     });
 
+    test('empty route name is ok', () {
+      final PathUrlStrategy strategy = PathUrlStrategy(location);
+      expect(strategy.prepareExternalUrl(''), '/');
+      expect(() => strategy.pushState(null, '', ''), returnsNormally);
+      expect(() => strategy.replaceState(null, '', ''), returnsNormally);
+    });
+
+    test('route names must start with /', () {
+      final PathUrlStrategy strategy = PathUrlStrategy(location);
+
+      expect(() => strategy.prepareExternalUrl('foo'), throwsAssertionError);
+      expect(() => strategy.prepareExternalUrl('foo/'), throwsAssertionError);
+      expect(() => strategy.prepareExternalUrl('foo/bar'), throwsAssertionError);
+
+      expect(() => strategy.pushState(null, '', 'foo'), throwsAssertionError);
+      expect(() => strategy.pushState(null, '', 'foo/'), throwsAssertionError);
+      expect(() => strategy.pushState(null, '', 'foo/bar'), throwsAssertionError);
+
+      expect(() => strategy.replaceState(null, '', 'foo'), throwsAssertionError);
+      expect(() => strategy.replaceState(null, '', 'foo/'), throwsAssertionError);
+      expect(() => strategy.replaceState(null, '', 'foo/bar'), throwsAssertionError);
+    });
+
     test('generates external path correctly in the presence of basePath', () {
       location.baseHref = 'https://example.com/foo/';
       final PathUrlStrategy strategy = PathUrlStrategy(location);
 
-      expect(strategy.prepareExternalUrl(''), '/foo');
+      expect(strategy.prepareExternalUrl(''), '/foo/');
       expect(strategy.prepareExternalUrl('/'), '/foo/');
-      expect(strategy.prepareExternalUrl('bar'), '/foo/bar');
       expect(strategy.prepareExternalUrl('/bar'), '/foo/bar');
       expect(strategy.prepareExternalUrl('/bar/'), '/foo/bar/');
     });