Fix more strong-mode warnings.

lib/src/multipart_file.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 import 'dart:async';
 import 'dart:convert';
 
+import 'package:async/async.dart';
 import 'package:http_parser/http_parser.dart';
 import 'package:path/path.dart' as path;
 
 import 'byte_stream.dart';
 import 'io.dart' as io;
 import 'utils.dart';
 
 /// A file to be uploaded as part of a [MultipartRequest]. This doesn't need to
 /// correspond to a physical file.
 class MultipartFile {
@@ skipping 61 matching lines @@
 
   // TODO(nweiz): Infer the content-type from the filename.
   /// Creates a new [MultipartFile] from a path to a file on disk.
   ///
   /// [filename] defaults to the basename of [filePath]. [contentType] currently
   /// defaults to `application/octet-stream`, but in the future may be inferred
   /// from [filename].
   ///
   /// This can only be used in an environment that supports "dart:io".
   static Future<MultipartFile> fromPath(String field, String filePath,
-      {String filename, MediaType contentType}) {
+      {String filename, MediaType contentType}) async {
     io.assertSupported("MultipartFile.fromPath");
     if (filename == null) filename = path.basename(filePath);
     var file = io.newFile(filePath);
-    return file.length().then((length) {
-      var stream = new ByteStream(file.openRead());
-      return new MultipartFile(field, stream, length,
-          filename: filename,
-          contentType: contentType);
-    });
+    var length = await file.length();
+    var stream = new ByteStream(DelegatingStream.typed(file.openRead()));
+    return new MultipartFile(field, stream, length,
+        filename: filename,
+        contentType: contentType);
   }
 
   // Finalizes the file in preparation for it being sent as part of a
   // [MultipartRequest]. This returns a [ByteStream] that should emit the body
   // of the file. The stream may be closed to indicate an empty file.
   ByteStream finalize() {
     if (isFinalized) {
       throw new StateError("Can't finalize a finalized MultipartFile.");
     }
     _isFinalized = true;
     return _stream;
   }
 }