[IO/http] - Fix for https://github.com/dart-lang/sdk/issues/49930

Check for null 'bufferedData' even in the path when 'subscription' is
null. Should also fix https://github.com/dart-lang/sdk/issues/26379

TEST=ci

Change-Id: Iacc18c8a38e2c5b36c5234495d39cf20bfd8bac3
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/330201
Reviewed-by: Brian Quinlan <bquinlan@google.com>
Commit-Queue: Siva Annamalai <asiva@google.com>

sdk/lib/_http/http_parser.dart

@@ -195,9 +195,9 @@ class _HttpDetachedIncoming extends Stream<Uint8List> {
       return _HttpDetachedStreamSubscription(subscription, bufferedData, onData)
         ..resume();
     } else {
-      // TODO(26379): add test for this branch.
-      return Stream<Uint8List>.fromIterable([bufferedData!]).listen(onData,
-          onError: onError, onDone: onDone, cancelOnError: cancelOnError);
+      return Stream<Uint8List>.fromIterable([bufferedData ?? Uint8List(0)])
+          .listen(onData,
+              onError: onError, onDone: onDone, cancelOnError: cancelOnError);
     }
   }
 }

tests/standalone/io/http_parser_header_add_test.dart

@@ -7,9 +7,13 @@
 
 import "dart:async";
 import "dart:io";
+// ignore: IMPORT_INTERNAL_LIBRARY
+import "dart:_http" show TestingClass$_HttpHeaders, TestingClass$_HttpParser;
 import "package:async_helper/async_helper.dart";
 import "package:expect/expect.dart";
 
+typedef _HttpParser = TestingClass$_HttpParser;
+
 Future<void> testFormatException() async {
   final server = await HttpServer.bind("127.0.0.1", 0);
   server.listen((HttpRequest request) {
@@ -28,6 +32,12 @@ Future<void> testFormatException() async {
   }
 }
 
+void testNullSubscriptionData() {
+  _HttpParser httpParser = new _HttpParser.requestParser();
+  httpParser.detachIncoming().listen((data) {}, onDone: () {});
+}
+
 main() {
   asyncTest(testFormatException);
+  testNullSubscriptionData();
 }