Always serialize 64-bit integers as strings in JSON.

test/json_test.dart

 // Basic smoke tests for the GeneratedMessage JSON API.
 //
 // There are more JSON tests in the dart-protoc-plugin package.
 library json_test;
 
 import 'dart:convert';
+import 'package:fixnum/fixnum.dart' show Int64;
 import 'package:test/test.dart';
 
 import 'mock_util.dart' show MockMessage, mockInfo;
 
 class T extends MockMessage {
   get info_ => _info;
   static final _info = mockInfo("T", () => new T());
 }
 
 main() {
@@ skipping 15 matching lines @@
     var t = new T();
     t.mergeFromJson('''{"1": 123, "2": "hello"}''');
     checkMessage(t);
   });
 
   test('testMergeFromJsonMap', () {
     var t = new T();
     t.mergeFromJsonMap({"1": 123, "2": "hello"});
     checkMessage(t);
   });
+
+  test('testInt64JsonEncoding', () {
+    final value = new Int64(1234567890123456789);
+    final t = new T()
+      ..int64 = value;
+    final encoded = t.writeToJsonMap();
+    expect(encoded["5"], "$value");
+    final decoded = new T()..mergeFromJsonMap(encoded);
+    expect(decoded.int64, value);
+  });
 }
 
 checkJsonMap(Map m) {
   expect(m.length, 2);
   expect(m["1"], 123);
   expect(m["2"], "hello");
 }
 
 checkMessage(T t) {
   expect(t.val, 123);
   expect(t.str, "hello");
 }