Always serialize 64-bit integers as strings in JSON.

test/mock_util.dart

 // Copyright(c) 2015, 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 mock_util;
 
+import 'package:fixnum/fixnum.dart' show Int64;
 import 'package:protobuf/protobuf.dart'
     show GeneratedMessage, BuilderInfo, CreateBuilderFunc, PbFieldType;
 
 BuilderInfo mockInfo(String className, CreateBuilderFunc create) {
   return new BuilderInfo(className)
     ..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.
 abstract class MockMessage extends GeneratedMessage {
   // subclasses must provide these
   BuilderInfo get info_;
 
   int get val => $_get(0, 1, 42);
   set val(x) => setField(1, x);
 
   String get str => $_get(1, 2, "");
   set str(x) => $_setString(1, 2, x);
 
   MockMessage get child => $_get(2, 3, null);
   set child(x) => setField(3, x);
 
   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);
   }
 }