Avoid parsing formal type parameters a second time from inside an async body,

tests/language/regress_27659_test.dart

+// Copyright (c) 2016, 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.
+
+const String lineLength = '120';
+
+foo({lineLength: lineLength}) {
+  print(lineLength);
+}
+
+bar({lineLength: lineLength}) async {
+  print(lineLength);
+}
+
+baz([lineLength = lineLength]) {
+  print(lineLength);
+}
+
+qux([lineLength = lineLength]) async {
+  print(lineLength);
+}
+
+main() {
+  foo();
+  bar();
+  baz();
+  qux();
+}