Fix look-ahead parsing code in the VM (fixes #29243).

tests/language/regress_29243_test.dart

+// 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.
+
+import "package:expect/expect.dart";
+
+int Function() x = () => 42;
+int Function(int Function()) y = (int Function() x) => x();
+List<int Function()> l = <int Function()>[()=>42, x];
+main() {
+  Expect.equals(42, y(l[1]));
+}