Use native async/await support in pub.

sdk/lib/_internal/pub_generated/test/upgrade/git/upgrade_to_nonexistent_pubspec_test.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.
-
-library pub_tests;
-
-import '../../descriptor.dart' as d;
-import '../../test_pub.dart';
-
-main() {
-  initConfig();
-  integration("upgrades Git packages to a nonexistent pubspec", () {
-    ensureGit();
-
-    var repo =
-        d.git('foo.git', [d.libDir('foo'), d.libPubspec('foo', '1.0.0')]);
-    repo.create();
-
-    d.appDir({
-      "foo": {
-        "git": "../foo.git"
-      }
-    }).create();
-
-    pubGet();
-
-    d.dir(
-        packagesPath,
-        [d.dir('foo', [d.file('foo.dart', 'main() => "foo";')])]).validate();
-
-    repo.runGit(['rm', 'pubspec.yaml']);
-    repo.runGit(['commit', '-m', 'delete']);
-
-    pubUpgrade(
-        error: new RegExp(
-            r'Could not find a file named "pubspec.yaml" ' r'in "[^\n]*"\.'));
-
-    d.dir(
-        packagesPath,
-        [d.dir('foo', [d.file('foo.dart', 'main() => "foo";')])]).validate();
-  });
-}