Rip out the last dart:io dependency from pkg/http.

pkg/http/test/io/multipart_test.dart

+// Copyright (c) 2014, 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.
+
+library http.test.io.multipart_test;
+
+import 'dart:async';
+import 'dart:io';
+
+import 'package:http/http.dart' as http;
+import 'package:path/path.dart' as path;
+import 'package:unittest/unittest.dart';
+
+import 'utils.dart';
+
+void main() {
+  var tempDir;
+  setUp(() {
+    tempDir = Directory.systemTemp.createTempSync('http_test_');
+  });
+
+  tearDown(() => tempDir.deleteSync(recursive: true));
+
+  test('with a file from disk', () {
+    expect(new Future.sync(() {
+      var filePath = path.join(tempDir.path, 'test-file');
+      new File(filePath).writeAsStringSync('hello');
+      return http.MultipartFile.fromPath('file', filePath);
+    }).then((file) {
+      var request = new http.MultipartRequest('POST', dummyUrl);
+      request.files.add(file);
+
+      expect(request, bodyMatches('''
+      --{{boundary}}
+      content-type: application/octet-stream
+      content-disposition: form-data; name="file"; filename="test-file"
+
+      hello
+      --{{boundary}}--
+      '''));
+    }), completes);
+  });
+}