Revert "Revert "Use native async/await support in pub.""

sdk/lib/_internal/pub_generated/test/snapshot/upgrades_snapshot_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 pub_tests;
-
-import 'package:path/path.dart' as p;
-import 'package:scheduled_test/scheduled_test.dart';
-
-import '../descriptor.dart' as d;
-import '../test_pub.dart';
-
-main() {
-  initConfig();
-  integration("upgrades a snapshot when its package is upgraded", () {
-    servePackages((builder) {
-      builder.serve(
-          "foo",
-          "1.2.3",
-          contents: [
-              d.dir("bin", [d.file("hello.dart", "void main() => print('hello!');")])]);
-    });
-
-    d.appDir({
-      "foo": "any"
-    }).create();
-
-    pubGet(output: contains("Precompiled foo:hello."));
-
-    d.dir(
-        p.join(appPath, '.pub', 'bin', 'foo'),
-        [d.matcherFile('hello.dart.snapshot', contains('hello!'))]).validate();
-
-    servePackages((builder) {
-      builder.serve(
-          "foo",
-          "1.2.4",
-          contents: [
-              d.dir("bin", [d.file("hello.dart", "void main() => print('hello 2!');")])]);
-    });
-
-    pubUpgrade(output: contains("Precompiled foo:hello."));
-
-    d.dir(
-        p.join(appPath, '.pub', 'bin', 'foo'),
-        [d.matcherFile('hello.dart.snapshot', contains('hello 2!'))]).validate();
-
-    var process = pubRun(args: ['foo:hello']);
-    process.stdout.expect("hello 2!");
-    process.shouldExit();
-  });
-}