Add type info to fix analyzer failures for 1.23.0-dev.5.0

lib/src/runner/loader.dart

Index: lib/src/runner/loader.dart
diff --git a/lib/src/runner/loader.dart b/lib/src/runner/loader.dart
index f24ddd0dd3b7b71ba284708b303077875966e812..23eea445fc1f7b7f49f05904a6e760d067decc2e 100644
--- a/lib/src/runner/loader.dart
+++ b/lib/src/runner/loader.dart
@@ -164,7 +164,9 @@ class Loader {
           return suite;
         } catch (error, stackTrace) {
           if (error is LoadException) rethrow;
-          await new Future.error(new LoadException(path, error), stackTrace);
+          await new Future.error(
+              new LoadException(path, error), stackTrace);
+          return null;
         }
       }, path: path, platform: platform);
     }