Allow constants as field initial values as well as creation thunks

lib/protobuf.dart

 // Copyright (c) 2011, 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 protobuf;
 
 import 'dart:collection' show ListMixin;
 import 'dart:convert' show JSON, Utf8Codec;
 import 'dart:math' as math;
 import 'dart:typed_data' show TypedData, Uint8List, ByteData, Endianness;
 
 import 'package:crypto/crypto.dart' show CryptoUtils;
 import 'package:fixnum/fixnum.dart' show Int64;
 
 part 'src/protobuf/coded_buffer_reader.dart';
 part 'src/protobuf/coded_buffer_writer.dart';
 part 'src/protobuf/builder_info.dart';
 part 'src/protobuf/exceptions.dart';
 part 'src/protobuf/extension.dart';
 part 'src/protobuf/extension_registry.dart';
 part 'src/protobuf/field_info.dart';
 part 'src/protobuf/generated_message.dart';
 part 'src/protobuf/pb_list.dart';
 part 'src/protobuf/protobuf_enum.dart';
 part 'src/protobuf/unknown_field_set.dart';
 part 'src/protobuf/utils.dart';
 part 'src/protobuf/wire_format.dart';
 
+// TODO(sra): Remove this method when clients upgrade to protoc 0.3.5
 Int64 makeLongInt(int n) => new Int64(n);
 
+// TODO(sra): Use Int64.parse() when available - see http://dartbug.com/21915.
+Int64 parseLongInt(String text) {
+  if (text.startsWith('0x')) return Int64.parseHex(text.substring(2));
+  if (text.startsWith('+0x')) return Int64.parseHex(text.substring(3));
+  if (text.startsWith('-0x')) return -Int64.parseHex(text.substring(3));
+  return Int64.parseInt(text);
+}
+
 const _UTF8 = const Utf8Codec(allowMalformed: true);