Add helper methods for skipping Dart VM native bodies.

pkg/front_end/lib/src/fasta/parser/dart_vm_native.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 Dart VM native method bodies on this form:
+///
+///     native STRING
+///
+/// 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 as in `dart2js`.
+library fasta.parser.dart_vm_native;
+
+import '../scanner/token.dart' show
+    Token;
+
+import '../scanner/token_constants.dart' show
+    STRING_TOKEN;
+
+import '../util/link.dart' show
+    Link;
+
+import 'parser.dart' show
+    optional;
+
+/// When parsing a Dart VM library file, we may encounter a native clause
+/// instead of a function body. This method skips such a clause.
+///
+/// This method is designed to be called when encountering
+/// [ErrorKind.ExpectedBlockToSkip] in [Listener.handleUnrecoverableError].
+Token skipNativeClause(Token token) {
+  if (!optional("native", token)) return null;
+  token = token.next;
+  if (token.kind != STRING_TOKEN) return null;
+  token = token.next;
+  if (!optional(";", token)) return null;
+  return token;
+}
+
+/// When parsing a Dart VM library file, we may encounter native getters like
+///
+///     int get length native "List_getLength";
+///
+/// This will result in [identifiers] being
+///
+///     [";", '"List_getLength"', "native", "length", "get"]
+///
+/// We need to remove '"List_getLength"' and "native" from that list.
+///
+/// This method designed to be called from [Listener.handleMemberName].
+Link<Token> removeNativeClause(Link<Token> identifiers) {
+  Link<Token> result = identifiers.tail;
+  if (result.head.kind != STRING_TOKEN) return identifiers;
+  result = result.tail;
+  if (result.isEmpty) return identifiers;
+  if (optional('native', result.head)) {
+    return result.tail.prepend(identifiers.head);
+  }
+  return identifiers;
+}