Revert "Remove unmodifiable map wrappers that could instead use UnmodifiableMapView."

pkg/yaml/lib/src/yaml_map.dart

Index: pkg/yaml/lib/src/yaml_map.dart
diff --git a/pkg/yaml/lib/src/yaml_map.dart b/pkg/yaml/lib/src/yaml_map.dart
index ab07c72b48d5aa98823159d954a10e5c8aa4d976..50ae2d95b942778ce5ec3e70492f30dfd83c7add 100644
--- a/pkg/yaml/lib/src/yaml_map.dart
+++ b/pkg/yaml/lib/src/yaml_map.dart
@@ -55,7 +55,7 @@ class YamlMap implements Map {
   /// Wraps an object for use as a key in the map.
   _wrapKey(obj) {
     if (obj != null && obj is! bool && obj is! List &&
-        (obj is! double || !obj.isNan) &&
+        (obj is! double || !obj.isNan()) &&
         (obj is! Map || obj is YamlMap)) {
       return obj;
     } else if (obj is Map) {