Include file path in JSON error when possible.

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 'dart:async';
 
 import 'package:unittest/unittest.dart';
 import 'package:yaml/yaml.dart';
@@ skipping 92 matching lines @@
         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 top level is not a map", () {
+        expect(() {
+          new LockFile.parse('''
+not a map
+''', sources);
+        }, throwsFormatException);
+      });
+
+      test("throws if the contents of 'packages' is not a map", () {
+        expect(() {
+          new LockFile.parse('''
+packages: not a map
+''', sources);
+        }, throwsFormatException);
+      });
+
       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'
             }
           }
         }));
       });
     });
   });
 }