Handle a null dependencies map in lockfile.

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

Index: sdk/lib/_internal/pub/test/lock_file_test.dart
diff --git a/sdk/lib/_internal/pub/test/lock_file_test.dart b/sdk/lib/_internal/pub/test/lock_file_test.dart
index 493741e5b89ad5192053d824b3a5d4da45d8b97f..b69cce4587c249b8484583376ce9fb2da55f47c9 100644
--- a/sdk/lib/_internal/pub/test/lock_file_test.dart
+++ b/sdk/lib/_internal/pub/test/lock_file_test.dart
@@ -89,6 +89,13 @@ packages:
         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('''