fix(dart_frog): revert Expando perf optimization

packages/dart_frog/lib/src/request.dart

@@ -110,7 +110,7 @@ class Request {
 
   Request._(this._request);
 
-  final shelf.Request _request;
+  shelf.Request _request;
 
   /// Connection information for the associated HTTP request.
   HttpConnectionInfo get connectionInfo {
@@ -146,12 +146,16 @@ class Request {
 
   /// Returns a [Future] containing the body as a [String].
   Future<String> body() async {
-    final bodyFromCache = _requestBodyCache[_request];
-    if (bodyFromCache != null) return bodyFromCache;
+    const requestBodyKey = 'dart_frog.request.body';
+    final bodyFromContext =
+        _request.context[requestBodyKey] as Completer<String>?;
+    if (bodyFromContext != null) return bodyFromContext.future;
 
     final completer = Completer<String>();
     try {
-      _requestBodyCache[_request] = completer.future;
+      _request = _request.change(
+        context: {..._request.context, requestBodyKey: completer},
+      );
       completer.complete(await _request.readAsString());
     } catch (error, stackTrace) {
       completer.completeError(error, stackTrace);
@@ -180,5 +184,3 @@ class Request {
     return Request._(_request.change(headers: headers, path: path, body: body));
   }
 }
-
-final _requestBodyCache = Expando<Future<String>>('dart_frog.request.body');

packages/dart_frog/test/src/serve_test.dart

@@ -27,6 +27,40 @@ void main() {
       await server.close();
     });
 
+    test('can read request.body across middleware:handler gap', () async {
+      Middleware middleware() {
+        return (handler) {
+          return (context) async {
+            await context.request.body(); // Read #1
+            return handler(context);
+          };
+        };
+      }
+
+      Handler handler() {
+        return (context) async {
+          await context.request.body(); // Read #2
+          return Response();
+        };
+      }
+
+      const address = 'localhost';
+      const port = 3002;
+      final pipeline = const Pipeline().addMiddleware(middleware());
+      final router = Router()..mount('/', handler());
+      final server = await serve(
+        pipeline.addHandler(router.call),
+        address,
+        port,
+      );
+      final client = HttpClient();
+      final request = await client.getUrl(Uri.parse('http://$address:$port'));
+      final response = await request.close();
+      expect(response.statusCode, equals(HttpStatus.ok));
+      client.close();
+      await server.close();
+    });
+
     test('exposes connectionInfo on the incoming request', () async {
       late HttpConnectionInfo connectionInfo;
       final server = await serve(