Add tests for parseAt with null values (dart-lang/yaml_edit#110)

pkgs/yaml_edit/test/parse_test.dart

diff --git a/pkgs/yaml_edit/test/parse_test.dart b/pkgs/yaml_edit/test/parse_test.dart
index d363090..ea2ae57 100644
--- a/pkgs/yaml_edit/test/parse_test.dart
+++ b/pkgs/yaml_edit/test/parse_test.dart
@@ -100,7 +100,7 @@
     test('with the correct value in nested collection', () {
       final doc = YamlEditor('''
 a: 1
-b: 
+b:
   d: 4
   e: [5, 6, 7]
 c: 3
@@ -109,6 +109,15 @@
       expect(doc.parseAt(['b', 'e', 2]).value, 7);
     });
 
+    test('with a null value in nested collection', () {
+      final doc = YamlEditor('''
+key1:
+  key2: null
+''');
+
+      expect(doc.parseAt(['key1', 'key2']).value, null);
+    });
+
     test('with the correct type (2)', () {
       final doc = YamlEditor("YAML: YAML Ain't Markup Language");
       final expectedYamlMap = doc.parseAt([]);
@@ -144,4 +153,14 @@
         ]).value,
         equals(4));
   });
+
+  test('works with null in path', () {
+    final doc = YamlEditor('{a: { ~: 4}}');
+    expect(doc.parseAt(['a', null]).value, equals(4));
+  });
+
+  test('works with null value', () {
+    final doc = YamlEditor('{a: null}');
+    expect(doc.parseAt(['a']).value, equals(null));
+  });
 }