Convert more pub code to use async/await.

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

Index: sdk/lib/_internal/pub/bin/async_compile.dart
diff --git a/sdk/lib/_internal/pub/bin/async_compile.dart b/sdk/lib/_internal/pub/bin/async_compile.dart
index a6f27c0fc6795230196037da8f7959ae68ed29cf..c8d56813a7cebb806ec3e0389745c85d1b377a9c 100644
--- a/sdk/lib/_internal/pub/bin/async_compile.dart
+++ b/sdk/lib/_internal/pub/bin/async_compile.dart
@@ -13,7 +13,7 @@ import 'package:path/path.dart' as p;
 /// 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.
 ///
@@ -172,4 +172,4 @@ void _writeFile(String path, String contents) {
   } on IOException catch (ex) {
     // Do nothing.
   }
-}
+}