Migrate the rest of the backend tests. (#2640)

* Migrate startAsyncUpload test.

* Migrate finishAsyncUpload test.

* Simplify test archive creation

Author: isoos

app/lib/frontend/testing/fake_upload_signer_service.dart

@@ -0,0 +1,33 @@
+// Copyright (c) 2019, 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 'package:pub_server/repository.dart';
+
+import '../upload_signer_service.dart';
+
+/// Returns an upload URL with no signed signature.
+class FakeUploadSignerService implements UploadSignerService {
+  final String _storagePrefix;
+  FakeUploadSignerService(this._storagePrefix);
+
+  @override
+  Future<AsyncUploadInfo> buildUpload(
+    String bucket,
+    String object,
+    Duration lifetime,
+    String successRedirectUrl, {
+    String predefinedAcl = 'project-private',
+    int maxUploadSize = UploadSignerService.maxUploadSize,
+  }) async {
+    return AsyncUploadInfo(Uri.parse('$_storagePrefix/$bucket/$object'), {
+      'key': '$bucket/$object',
+      'success_action_redirect': successRedirectUrl,
+    });
+  }
+
+  @override
+  Future<SigningResult> sign(List<int> bytes) async {
+    return SigningResult('google-access-id', [1, 2, 3, 4]);
+  }
+}