Handle empty maps when converting YAML to a string.

sdk/lib/_internal/pub/test/utils_test.dart

 // Copyright (c) 2012, 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.
 
 library utils_test;
 
 import 'package:unittest/unittest.dart';
 import 'test_pub.dart';
 import '../lib/src/utils.dart';
 
@@ skipping 62 matching lines @@
       map[null] = "null";
       map[123] = "num";
       map[true] = "bool";
 
       expect(yamlToString(map),
           equals("""
 123: num
 null: null
 true: bool"""));
     });
+
+    test('handles empty maps', () {
+      expect(yamlToString({}), equals("{}"));
+      expect(yamlToString({'a': {}, 'b': {}}), equals("""
+a: {}
+b: {}"""));
+    });
   });
 }