Use the cached pubspec if possible for describing hosted packages.

utils/tests/pub/version_solver_test.dart

Index: utils/tests/pub/version_solver_test.dart
diff --git a/utils/tests/pub/version_solver_test.dart b/utils/tests/pub/version_solver_test.dart
index d4d85a68b8444eda2cf6053c7c7e754767ed8052..69287c78a413d45f37e71dc245b52b0dd7eb974e 100644
--- a/utils/tests/pub/version_solver_test.dart
+++ b/utils/tests/pub/version_solver_test.dart
@@ -868,6 +868,10 @@ class MockSource extends Source {
 
   MockSource(this.name);
 
+  Future<String> systemCacheDirectory(PackageId id) {
+    return new Future.value('${id.name}-${id.version}');
+  }
+
   Future<List<Version>> getVersions(String name, String description) {
     return new Future.sync(() {
       // Make sure the solver doesn't request the same thing twice.