Fixed type cast error

[riccardoratta]

When running the example repository on dart 2.0.0 I always get:

Asynchronous error
type 'List<dynamic>' is not a subtype of type 'List<Map<dynamic, dynamic>>' in type cast
dart:core                                              Object._as
example/src/examples/http_proxy_repository.dart 31:37  HttpProxyRepository.versions
===== asynchronous gap ===========================
example/src/examples/cow_repository.dart 135:18        _RemoteMetadataCache.fetchVersionlist.<fn>
dart:collection                                        __InternalLinkedHashMap&_HashVMBase&MapMixin&_LinkedHashMapMixin.putIfAbsent
example/src/examples/cow_repository.dart 131:10        _RemoteMetadataCache.fetchVersionlist
example/src/examples/cow_repository.dart 47:35         CopyAndWriteRepository.versions.onListen
dart:async                                             Stream.toList
package:pub_server/shelf_pubserver.dart 242:62         ShelfPubServer._listVersions

The change proposed fixed the problem.

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