Add test to validate SourceSpan values for nodes

Preparing for https://github.com/dart-lang/yaml/issues/51

pkgs/yaml/test/span_test.dart

diff --git a/pkgs/yaml/test/span_test.dart b/pkgs/yaml/test/span_test.dart
new file mode 100644
index 0000000..7ef61d3
--- /dev/null
+++ b/pkgs/yaml/test/span_test.dart
@@ -0,0 +1,89 @@
+// Copyright (c) 2019, the Dart project authors. Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+import 'dart:convert';
+
+import 'package:source_span/source_span.dart';
+import 'package:test/test.dart';
+import 'package:yaml/yaml.dart';
+
+void _expectSpan(SourceSpan source, String expected) {
+  final result = source.message('message');
+  printOnFailure("r'''\n$result'''");
+
+  expect(result, expected);
+}
+
+void main() {
+  YamlMap yaml;
+
+  setUpAll(() {
+    yaml = loadYaml(const JsonEncoder.withIndent(' ').convert({
+      'num': 42,
+      'nested': {
+        'null': null,
+        'num': 42,
+      },
+      'null': null,
+    })) as YamlMap;
+  });
+
+  test('first root key', () {
+    _expectSpan(
+      yaml.nodes['num'].span,
+      r'''
+line 2, column 9: message
+  ╷
+2 │  "num": 42,
+  │         ^^
+  ╵''',
+    );
+  });
+
+  test('first root key', () {
+    _expectSpan(
+      yaml.nodes['null'].span,
+      r'''
+line 7, column 10: message
+  ╷
+7 │  "null": null
+  │          ^^^^
+  ╵''',
+    );
+  });
+
+  group('nested', () {
+    YamlMap nestedMap;
+
+    setUpAll(() {
+      nestedMap = yaml.nodes['nested'] as YamlMap;
+    });
+
+    test('first root key', () {
+      _expectSpan(
+        nestedMap.nodes['null'].span,
+        r'''
+line 4, column 11: message
+  ╷
+4 │   "null": null,
+  │           ^^^^
+  ╵''',
+      );
+    });
+
+    test('first root key', () {
+      _expectSpan(
+        nestedMap.nodes['num'].span,
+        r'''
+line 5, column 10: message
+  ╷
+5 │     "num": 42
+  │ ┌──────────^
+6 │ │  },
+  │ └─^
+  ╵''',
+      );
+    });
+  });
+}