Convert more pub code to use async/await.

sdk/lib/_internal/pub/bin/async_compile.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:io';
 
 import 'package:async_await/async_await.dart' as async_await;
 import 'package:path/path.dart' as p;
 
 /// A changing string that indicates the "version" or timestamp of the compiler
 /// that the current sources were compiled against.
 ///
 /// Increment this whenever a meaningful change in the async/await compiler
 /// itself is landed. Bumping this will force all previously compiled files
 /// that were compiled against an older compiler to be recompiled.
-const COMPILER_VERSION = "1";
+const COMPILER_VERSION = "2";
 
 /// The path to pub's root directory (sdk/lib/_internal/pub) in the Dart repo.
 ///
 /// This assumes this script is itself being run from within the repo.
 final sourceDir = p.dirname(p.dirname(p.fromUri(Platform.script)));
 
 /// The [sourceDir] as a URL, for use in import strings.
 final sourceUrl = p.toUri(sourceDir).toString();
 
 /// The directory that compiler output should be written to.
@@ skipping 138 matching lines @@
 /// Writes [contents] to [path], ignoring any IO errors that occur.
 ///
 /// This swallows errors to accommodate multiple compilers running concurrently.
 /// Since they will produce the same output anyway, a failure of one is fine.
 void _writeFile(String path, String contents) {
   try {
     new File(path).writeAsStringSync(contents);
   } on IOException catch (ex) {
     // Do nothing.
   }
-}
+}