[flutter_tools] ensure etag headers are ascii (#55704)

packages/flutter_tools/lib/src/build_runner/devfs_web.dart

@@ -350,7 +350,8 @@ class WebAssetServer implements AssetReader {
 
     // For real files, use a serialized file stat plus path as a revision.
     // This allows us to update between canvaskit and non-canvaskit SDKs.
-    final String etag = file.lastModifiedSync().toIso8601String() + file.path;
+    final String etag = file.lastModifiedSync().toIso8601String()
+      + Uri.encodeComponent(file.path);
     if (ifNoneMatch == etag) {
       return shelf.Response.notModified();
     }

packages/flutter_tools/test/general.shard/web/devfs_web_test.dart

@@ -324,6 +324,18 @@ void main() {
     expect((await response.read().toList()).first, source.readAsBytesSync());
   }));
 
+  test('serves valid etag header for asset files with non-ascii chracters', () => testbed.run(() async {
+    globals.fs.file(globals.fs.path.join('build', 'flutter_assets', 'fooπ'))
+      ..createSync(recursive: true)
+      ..writeAsBytesSync(<int>[1, 2, 3]);
+
+    final Response response = await webAssetServer
+      .handleRequest(Request('GET', Uri.parse('http://foobar/assets/fooπ')));
+    final String etag = response.headers[HttpHeaders.etagHeader];
+
+    expect(etag.runes, everyElement(predicate((int char) => char < 255)));
+  }));
+
   test('handles serving missing asset file', () => testbed.run(() async {
     final Response response = await webAssetServer
       .handleRequest(Request('GET', Uri.parse('http://foobar/assets/foo')));