Move parser helper classes to own files and clean them up.

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

Index: pkg/front_end/lib/src/fasta/parser/util.dart
diff --git a/pkg/front_end/lib/src/fasta/parser/util.dart b/pkg/front_end/lib/src/fasta/parser/util.dart
new file mode 100644
index 0000000000000000000000000000000000000000..f5404ce56a4c112b7d3c689e7018747e923d177c
--- /dev/null
+++ b/pkg/front_end/lib/src/fasta/parser/util.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.util;
+
+import '../scanner.dart' show Token;
+
+/// Returns true if [token] is the symbol or keyword [value].
+bool optional(String value, Token token) {
+  return identical(value, token.stringValue);
+}