Always serialize 64-bit integers as strings in JSON.

test/map_mixin_test.dart

Index: test/map_mixin_test.dart
diff --git a/test/map_mixin_test.dart b/test/map_mixin_test.dart
index b732a96236a557544320f944540c6bf3350dc2b7..e3d807e8443353512ae2c577d6c8460ebefbcf8a 100644
--- a/test/map_mixin_test.dart
+++ b/test/map_mixin_test.dart
@@ -29,7 +29,7 @@ main() {
 
     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"], "");
@@ -38,10 +38,11 @@ main() {
     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', () {