Fix all strong mode warnings.

lib/src/utils.dart

Index: lib/src/utils.dart
diff --git a/lib/src/utils.dart b/lib/src/utils.dart
index 5781d5f0ab12a227e861f4fd23103f4cc64c05ab..710abdc04a4813ccd94154c71ac3774401ffc871 100644
--- a/lib/src/utils.dart
+++ b/lib/src/utils.dart
@@ -19,20 +19,3 @@ String padRight(String string, int length) {
 
   return result.toString();
 }
-
-/// Flattens nested lists inside an iterable into a single list containing only
-/// non-list elements.
-List flatten(Iterable nested) {
-  var result = [];
-  helper(list) {
-    for (var element in list) {
-      if (element is List) {
-        helper(element);
-      } else {
-        result.add(element);
-      }
-    }
-  }
-  helper(nested);
-  return result;
-}