Always serialize 64-bit integers as strings in JSON.

test/mock_util.dart

Index: test/mock_util.dart
diff --git a/test/mock_util.dart b/test/mock_util.dart
index a5ac749f966db38e655809c5b7ff5831fd7f40ea..a0a36d8d25261803fd7eee66b6e8f72ebb1b8f39 100644
--- a/test/mock_util.dart
+++ b/test/mock_util.dart
@@ -4,6 +4,7 @@
 
 library mock_util;
 
+import 'package:fixnum/fixnum.dart' show Int64;
 import 'package:protobuf/protobuf.dart'
     show GeneratedMessage, BuilderInfo, CreateBuilderFunc, PbFieldType;
 
@@ -12,7 +13,8 @@ BuilderInfo mockInfo(String className, CreateBuilderFunc create) {
     ..a(1, "val", PbFieldType.O3, 42)
     ..a(2, "str", PbFieldType.OS)
     ..a(3, "child", PbFieldType.OM, create, create)
-    ..p(4, "int32s", PbFieldType.P3);
+    ..p(4, "int32s", PbFieldType.P3)
+    ..a(5, "int64", PbFieldType.O6);
 }
 
 /// A minimal protobuf implementation for testing.
@@ -31,6 +33,9 @@ abstract class MockMessage extends GeneratedMessage {
 
   List<int> get int32s => $_get(3, 4, null);
 
+  Int64 get int64 => $_get(4, 5, 0);
+  set int64(x) => setField(5, x);
+
   clone() {
     CreateBuilderFunc create = info_.byName["child"].subBuilder;
     return create()..mergeFromMessage(this);