Support a user-supplied dart:io client in pkg/http.

pkg/http/lib/src/io.dart

Index: pkg/http/lib/src/io.dart
diff --git a/pkg/http/lib/src/io.dart b/pkg/http/lib/src/io.dart
index 1730706331f41b112334f40876e13c4e1b787d6b..767dda054a2f61638e5eecaaf82220e3ae35e0c5 100644
--- a/pkg/http/lib/src/io.dart
+++ b/pkg/http/lib/src/io.dart
@@ -43,6 +43,9 @@ newFile(String path) => _file.newInstance(const Symbol(''), [path]).reflectee;
 /// Returns whether [error] is a `dart:io` HttpException.
 bool isHttpException(error) => reflect(error).type.isSubtypeOf(_httpException);
 
+/// Returns whether [client] is a `dart:io` HttpClient.
+bool isHttpClient(client) => reflect(client).type.isSubtypeOf(_httpClient);
+
 /// Tries to load `dart:io` and returns `null` if it fails.
 LibraryMirror _getLibrary() {
   try {