Fix all strong-mode errors and warnings.

lib/src/runner/plugin/platform_helpers.dart

Index: lib/src/runner/plugin/platform_helpers.dart
diff --git a/lib/src/runner/plugin/platform_helpers.dart b/lib/src/runner/plugin/platform_helpers.dart
index 8a8593408b6841636a8d4bd114df894b9329c85d..50d081280885038687f9a29b3e6019a996650e44 100644
--- a/lib/src/runner/plugin/platform_helpers.dart
+++ b/lib/src/runner/plugin/platform_helpers.dart
@@ -113,9 +113,10 @@ class _Deserializer {
   /// Deserializes [group] into a concrete [Group].
   Group deserializeGroup(Map group) {
     var metadata = new Metadata.deserialize(group['metadata']);
-    return new Group(group['name'], group['entries'].map((entry) {
-      if (entry['type'] == 'group') return deserializeGroup(entry);
-      return _deserializeTest(entry);
+    return new Group(group['name'], (group['entries'] as List).map((entry) {
+      var map = entry as Map;
+      if (map['type'] == 'group') return deserializeGroup(map);
+      return _deserializeTest(map);
     }),
         metadata: metadata,
         setUpAll: _deserializeTest(group['setUpAll']),