Use native async/await support in pub.

sdk/lib/_internal/pub_generated/test/global/binstubs/utils.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.
-
-import 'dart:async';
-import 'dart:io';
-
-import 'package:path/path.dart' as p;
-
-import '../../test_pub.dart';
-
-/// The buildbots do not have the Dart SDK (containing "dart" and "pub") on
-/// their PATH, so we need to spawn the binstub process with a PATH that
-/// explicitly includes it.
-Future<Map> getEnvironment() {
-  final completer0 = new Completer();
-  scheduleMicrotask(() {
-    try {
-      var binDir = p.dirname(Platform.executable);
-      join0(x0) {
-        var separator = x0;
-        var path = "${Platform.environment["PATH"]}${separator}${binDir}";
-        new Future.value(getPubTestEnvironment()).then((x1) {
-          try {
-            var environment = x1;
-            environment["PATH"] = path;
-            completer0.complete(environment);
-          } catch (e0, s0) {
-            completer0.completeError(e0, s0);
-          }
-        }, onError: completer0.completeError);
-      }
-      if (Platform.operatingSystem == "windows") {
-        join0(";");
-      } else {
-        join0(":");
-      }
-    } catch (e, s) {
-      completer0.completeError(e, s);
-    }
-  });
-  return completer0.future;
-}