Make _MAP_PROPERTIES a Set rather than a List

Reminded about this by Kasper Lund’s dart-archive#614.

Author: chalin

lib/core/parser/utils.dart

@@ -97,12 +97,12 @@ setKeyed(object, key, value) {
   return value;
 }
 
-final List<String> _MAP_PROPERTIES = const <String>[
+final Set<String> _MAP_PROPERTIES = new Set<String>.from([
   "hashCode", // any reason to exclude this?
   "isEmpty",
   "isNotEmpty",
   "keys",
   "length",
   "runtimeType", // any reason to exclude this?
-  "values"];
+  "values"]);
 bool isMapProperty(String key) => _MAP_PROPERTIES.contains(key);