Handle a null dependencies map in lockfile.

sdk/lib/_internal/pub/test/lock_file_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 lock_file_test;
 
 import 'package:unittest/unittest.dart';
 import 'package:yaml/yaml.dart';
 
 import '../lib/src/lock_file.dart';
@@ skipping 71 matching lines @@
 packages:
   foo:
     source: bad
     version: 1.2.3
     description: foo desc
 ''', sources);
         var foo = lockFile.packages['foo'];
         expect(foo.source, equals('bad'));
       });
 
+      test("allows an empty dependency map", () {
+        var lockFile = new LockFile.parse('''
+packages:
+''', sources);
+        expect(lockFile.packages, isEmpty);
+      });
+
       test("throws if the version is missing", () {
         expect(() {
           new LockFile.parse('''
 packages:
   foo:
     source: mock
     description: foo desc
 ''', sources);
         }, throwsFormatException);
       });
@@ skipping 81 matching lines @@
               'version': '3.2.1',
               'source': 'mock',
               'description': 'bar desc'
             }
           }
         }));
       });
     });
   });
 }