Track async state in parser.

pkg/front_end/lib/src/fasta/parser/async_modifier.dart

Index: pkg/front_end/lib/src/fasta/parser/async_modifier.dart
diff --git a/pkg/front_end/lib/src/fasta/parser/async_modifier.dart b/pkg/front_end/lib/src/fasta/parser/async_modifier.dart
new file mode 100644
index 0000000000000000000000000000000000000000..0c09d23eaec4e09fc671d37ef634594f4b71c7dd
--- /dev/null
+++ b/pkg/front_end/lib/src/fasta/parser/async_modifier.dart
@@ -0,0 +1,12 @@
+// Copyright (c) 2017, 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.
+
+library fasta.parser.async_modifier;
+
+enum AsyncModifier {
+  Sync,
+  SyncStar,
+  Async,
+  AsyncStar,
+}