Remove an unnecessary test expectation (#42)

This expect is testing language implementation which is not necessary to
do in this package.
diff --git a/test/parser_test.dart b/test/parser_test.dart
index 45ac52d..1b73f13 100644
--- a/test/parser_test.dart
+++ b/test/parser_test.dart
@@ -357,8 +357,6 @@
       mapping = parseJsonExtended(expected);
       expect(mapping.toJson(), equals(expected));
     }
-    // Invalid for this case
-    expect(() => parseJson(SOURCE_MAP_BUNDLE as dynamic), throwsA(anything));
 
     var mapping = parseJsonExtended(SOURCE_MAP_BUNDLE) as MappingBundle;
     expect(mapping.toJson(), equals(SOURCE_MAP_BUNDLE));