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

pkg/front_end/lib/src/fasta/parser/native_support.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.
 
 /// Implements support for two variants of the native syntax extension.
 ///
 ///   * The Dart VM variant, where native method bodies have this form:
 ///
 ///     methodDeclaration() native STRING;
 ///
 ///   * The Dart2js and DDC variant, where native method bodies have this form:
 ///
 ///     methodDeclaration() native;
 ///
 /// This support is kept separate from parser.dart as this isn't specified in
 /// the Dart Language Specification, also we hope to remove this syntax long
 /// term and replace it with annotations and external declarations.
 library fasta.parser.dart_vm_native;
 
 import '../../scanner/token.dart' show Token;
 
+import '../quote.dart' show unescapeString;
+
 import '../scanner/token_constants.dart' show STRING_TOKEN;
 
 import '../util/link.dart' show Link;
 
-import 'parser.dart' show optional;
-
-import '../quote.dart' show unescapeString;
+import 'util.dart' show optional;
 
 /// When parsing a library file, we may encounter a native clause
 /// instead of a function body. This method skips such a clause. The
 /// [expectString] argument is used to choose which variant of the native clause
 /// we expect to parse.
 ///
 /// This method is designed to be called when encountering
 /// [ErrorKind.ExpectedBlockToSkip] in [Listener.handleUnrecoverableError].
 Token skipNativeClause(Token token, bool expectString) {
   if (!optional("native", token)) return null;
@@ skipping 38 matching lines @@
   }
   return identifiers;
 }
 
 /// When the parser encounters a native clause and expects a string (like in VM
 /// and flutter patch files), this method extracts the native name in that
 /// string.
 String extractNativeMethodName(Token token) {
   return unescapeString(token.next.lexeme);
 }