Always serialize 64-bit integers as strings in JSON.

test/map_mixin_test.dart

 #!/usr/bin/env 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.
 
 // Unit tests for PbMapMixin.
 // There are more tests in the dart-protoc-plugin package.
 library map_mixin_test;
 
 import 'dart:collection' show MapMixin;
@@ skipping 11 matching lines @@
   @override
   String toString() => "Rec(${val}, \"${str}\")";
 }
 
 main() {
   test('PbMapMixin methods return default field values', () {
     var r = new Rec();
 
     expect(r.isEmpty, false);
     expect(r.isNotEmpty, true);
-    expect(r.keys, ["val", "str", "child", "int32s"]);
+    expect(r.keys, ["val", "str", "child", "int32s", "int64"]);
 
     expect(r["val"], 42);
     expect(r["str"], "");
     expect(r["child"].runtimeType, Rec);
     expect(r["child"].toString(), 'Rec(42, "")');
     expect(r["int32s"], []);
 
     var v = r.values;
-    expect(v.length, 4);
+    expect(v.length, 5);
     expect(v.first, 42);
     expect(v.toList()[1], "");
-    expect(v.last.toString(), '[]');
+    expect(v.toList()[3].toString(), '[]');
+    expect(v.last, 0);
   });
 
   test('operator []= sets record fields', () {
     var r = new Rec();
 
     r["val"] = 123;
     expect(r.val, 123);
     expect(r["val"], 123);
 
     r["str"] = "hello";
@@ skipping 42 matching lines @@
   });
 
   test("reading protobuf values shouldn't change equality", () {
     var a = new Rec();
     var b = new Rec();
     expect(a == b, true);
     new Map.from(a);
     expect(a == b, true);
   });
 }