Allow constants as field initial values as well as creation thunks

lib/protobuf.dart

Index: lib/protobuf.dart
diff --git a/lib/protobuf.dart b/lib/protobuf.dart
index f1e55cd21534cb6604e5d7a8a2d2a4e76460a555..d74c9c8b813b52e8f66dd75c7d847f5c3363906c 100644
--- a/lib/protobuf.dart
+++ b/lib/protobuf.dart
@@ -26,6 +26,15 @@ 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);