Regenerate pub using the latest version of async-await.

sdk/lib/_internal/pub_generated/bin/pub.dart

 // Copyright (c) 2013, 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:args/args.dart';
 import 'package:http/http.dart' as http;
 import 'package:path/path.dart' as path;
@@ skipping 235 matching lines @@
 
 /// Checks that pub is running on a supported platform.
 ///
 /// If it isn't, it prints an error message and exits. Completes when the
 /// validation is done.
 Future validatePlatform() {
   final completer0 = new Completer();
   scheduleMicrotask(() {
     try {
       join0() {
-        runProcess('ver', []).then((x0) {
+        new Future.value(runProcess('ver', [])).then((x0) {
           try {
             var result = x0;
             join1() {
               completer0.complete();
             }
             if (result.stdout.join('\n').contains('XP')) {
               log.error('Sorry, but pub is not supported on Windows XP.');
-              flushThenExit(exit_codes.USAGE).then((x1) {
+              new Future.value(flushThenExit(exit_codes.USAGE)).then((x1) {
                 try {
                   x1;
                   join1();
                 } catch (e0, s0) {
                   completer0.completeError(e0, s0);
                 }
               }, onError: completer0.completeError);
             } else {
               join1();
             }
           } catch (e1, s1) {
             completer0.completeError(e1, s1);
           }
         }, onError: completer0.completeError);
       }
       if (Platform.operatingSystem != 'windows') {
         completer0.complete(null);
       } else {
         join0();
       }
     } catch (e, s) {
       completer0.completeError(e, s);
     }
   });
   return completer0.future;
 }