Fix JSON encoder accepting NaN and Infinity.

pkg/json/test/json_test.dart

Index: pkg/json/test/json_test.dart
diff --git a/pkg/json/test/json_test.dart b/pkg/json/test/json_test.dart
index d146ce38de9e337daa6c3ccec7d4dad06684d039..c3aaf7028b89d3d1bd81a2c10efd17bf1def1580 100644
--- a/pkg/json/test/json_test.dart
+++ b/pkg/json/test/json_test.dart
@@ -123,6 +123,17 @@ main() {
       json.stringify([new Object()]);
     }, throwsJsonError);
 
+    expect(() {
+      json.stringify([double.NAN]);
+    }, throwsJsonError);
+
+    expect(() {
+      json.stringify([double.INFINITY]);
+    }, throwsJsonError);
+
+    expect(() {
+      json.stringify([-double.INFINITY]);
+    }, throwsJsonError);
   });
 
   test('stringify throws if argument cannot be converted', () {