Convert more pub code to use async/await.

sdk/lib/_internal/pub/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 111 matching lines @@
   } else if (exception is FormatException || exception is DataException) {
     return exit_codes.DATA;
   } else if (exception is UsageException) {
     return exit_codes.USAGE;
   } else {
     return 1;
   }
 }
 
 /// Walks the command tree and runs the selected pub command.
-Future invokeCommand(String cacheDir, ArgResults mainOptions) {
+Future invokeCommand(String cacheDir, ArgResults mainOptions) async {
   var commands = PubCommand.mainCommands;
   var command;
   var commandString = "pub";
   var options = mainOptions;
 
   while (commands.isNotEmpty) {
     if (options.command == null) {
       if (options.rest.isEmpty) {
         if (command == null) {
           // No top-level command was chosen.
@@ skipping 24 matching lines @@
       return new Future.value();
     }
   }
 
   // Make sure there aren't unexpected arguments.
   if (!command.takesArguments && options.rest.isNotEmpty) {
     command.usageError(
         'Command "${options.name}" does not take any arguments.');
   }
 
-  return syncFuture(() {
+  try {
+    // TODO(rnystrom): Use await here when this is fixed:
+    // https://github.com/dart-lang/async_await/issues/40.
     return command.run(cacheDir, mainOptions, options);
-  }).whenComplete(() {
+  } finally {
     command.cache.deleteTempDir();
-  });
+  }
 }
 
 /// 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() {
-  return syncFuture(() {
-    if (Platform.operatingSystem != 'windows') return null;
+Future validatePlatform() async {
+  if (Platform.operatingSystem != 'windows') return;
 
-    return runProcess('ver', []).then((result) {
-      if (result.stdout.join('\n').contains('XP')) {
-        log.error('Sorry, but pub is not supported on Windows XP.');
-        return flushThenExit(exit_codes.USAGE);
-      }
-    });
-  });
+  var result = await runProcess('ver', []);
+  if (result.stdout.join('\n').contains('XP')) {
+    log.error('Sorry, but pub is not supported on Windows XP.');
+    await flushThenExit(exit_codes.USAGE);
+  }
 }