[web] Fix window.defaultRouteName (flutter/engine#17580)

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

@@ -9,27 +9,6 @@ part of engine;
 
 // Some parts of this file were inspired/copied from the AngularDart router.
 
-/// Ensures that `str` is prefixed with `leading`.
-///
-/// If `str` is already prefixed, it'll be returned unchanged. If it's not,
-/// this function will prefix it.
-///
-/// The `applyWhenEmpty` flag controls whether this function should prefix `str`
-/// or not when it's an empty string.
-///
-/// ```dart
-/// ensureLeading('/path', '/'); // "/path"
-/// ensureLeading('path', '/'); // "/path"
-/// ensureLeading('', '/'); // "/"
-/// ensureLeading('', '/', applyWhenEmpty: false); // ""
-/// ```
-String ensureLeading(String str, String leading, {bool applyWhenEmpty = true}) {
-  if (str.isEmpty && !applyWhenEmpty) {
-    return str;
-  }
-  return str.startsWith(leading) ? str : '$leading$str';
-}
-
 /// [LocationStrategy] is responsible for representing and reading route state
 /// from the browser's URL.
 ///
@@ -93,12 +72,11 @@ class HashLocationStrategy extends LocationStrategy {
     // the hash value is always prefixed with a `#`
     // 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.
-    path = path.isEmpty ? path : path.substring(1);
-    // The path, by convention, should always contain a leading '/'.
-    return ensureLeading(path, '/');
+    return path.isEmpty ? path : path.substring(1);
   }
 
   @override

engine/src/flutter/lib/web_ui/lib/src/engine/test_embedding.dart

@@ -37,7 +37,7 @@ class TestLocationStrategy extends LocationStrategy {
         history = <TestHistoryEntry>[initialEntry];
 
   @override
-  String get path => ensureLeading(currentEntry.url, '/');
+  String get path => currentEntry.url;
 
   int _currentEntryIndex;
   int get currentEntryIndex => _currentEntryIndex;

engine/src/flutter/lib/web_ui/lib/src/engine/window.dart

@@ -441,6 +441,10 @@ class EngineWindow extends ui.Window {
             _replyToPlatformMessage(callback, codec.encodeSuccessEnvelope(true));
             break;
         }
+        // As soon as Flutter starts taking control of the app navigation, we
+        // should reset [_defaultRouteName] to "/" so it doesn't have any
+        // further effect after this point.
+        _defaultRouteName = '/';
         return;
     }
 

engine/src/flutter/lib/web_ui/test/engine/history_test.dart

@@ -3,7 +3,7 @@
 // found in the LICENSE file.
 
 // @dart = 2.6
-@TestOn('vm && linux')
+@TestOn('!safari')
 // TODO(nurhan): https://github.com/flutter/flutter/issues/51169
 
 import 'dart:async';

engine/src/flutter/lib/web_ui/test/window_test.dart

@@ -3,9 +3,15 @@
 // found in the LICENSE file.
 
 // @dart = 2.6
+import 'dart:typed_data';
+
 import 'package:test/test.dart';
 import 'package:ui/src/engine.dart';
 
+const MethodCodec codec = JSONMethodCodec();
+
+void emptyCallback(ByteData date) {}
+
 TestLocationStrategy _strategy;
 TestLocationStrategy get strategy => _strategy;
 set strategy(TestLocationStrategy newStrategy) {
@@ -17,7 +23,27 @@ void main() {
     strategy = TestLocationStrategy.fromEntry(TestHistoryEntry('initial state', null, '/initial'));
     expect(window.defaultRouteName, '/initial');
 
+    // Changing the URL in the address bar later shouldn't affect [window.defaultRouteName].
     strategy.replaceState(null, null, '/newpath');
     expect(window.defaultRouteName, '/initial');
   });
+
+  test('window.defaultRouteName should reset after navigation platform message', () {
+    strategy = TestLocationStrategy.fromEntry(TestHistoryEntry('initial state', null, '/initial'));
+    // Reading it multiple times should return the same value.
+    expect(window.defaultRouteName, '/initial');
+    expect(window.defaultRouteName, '/initial');
+
+    window.sendPlatformMessage(
+      'flutter/navigation',
+      JSONMethodCodec().encodeMethodCall(MethodCall(
+        'routePushed',
+        <String, dynamic>{'previousRouteName': '/foo', 'routeName': '/bar'},
+      )),
+      emptyCallback,
+    );
+    // After a navigation platform message, [window.defaultRouteName] should
+    // reset to "/".
+    expect(window.defaultRouteName, '/');
+  });
 }