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

pkg/front_end/lib/src/fasta/parser/formal_parameter_kind.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.
+
+library fasta.parser.formal_parameter_kind;
+
+enum FormalParameterKind {
+  mandatory,
+  optionalNamed,
+  optionalPositional,
+}
+
+bool isMandatoryFormalParameterKind(FormalParameterKind type) {
+  return FormalParameterKind.mandatory == type;
+}
+
+bool isOptionalNamedFormalParameterKind(FormalParameterKind type) {
+  return FormalParameterKind.optionalNamed == type;
+}
+
+bool isOptionalPositionalFormalParameterKind(FormalParameterKind type) {
+  return FormalParameterKind.optionalPositional == type;
+}