Update the protocol buffer library to use Int64 from the fixnum package

lib/pb_list.dart

Index: lib/pb_list.dart
diff --git a/lib/pb_list.dart b/lib/pb_list.dart
index a1dc4142cf7882d1bea630531b9082388fede4ef..2ba1b06ff90dc215135e3824d36120a9c79947e7 100644
--- a/lib/pb_list.dart
+++ b/lib/pb_list.dart
@@ -167,8 +167,8 @@ class PbUint32List extends PbList<int> {
  * A [PbList] that requires its elements to be [int]s in the range
  * [:2^-63, 2^63 - 1:].
  */
-class PbSint64List extends PbList<ByteData> {
-  void _validateElement(ByteData val) {
+class PbSint64List extends PbList<Int64> {
+  void _validateElement(Int64 val) {
     if (!_isSigned64(val)) {
       throw new ArgumentError('Illegal to add value (${val}):'
           ' out of range for sint64');
@@ -180,8 +180,8 @@ class PbSint64List extends PbList<ByteData> {
  * A [PbList] that requires its elements to be [int]s in the range
  * [:0, 2^64 - 1:].
  */
-class PbUint64List extends PbList<ByteData> {
-  void _validateElement(ByteData val) {
+class PbUint64List extends PbList<Int64> {
+  void _validateElement(Int64 val) {
     if (!_isUnsigned64(val)) {
       throw new ArgumentError('Illegal to add value (${val}):'
           ' out of range for uint64');