Fix all strong mode warnings.

lib/src/utils.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 /// The line used in the string representation of stack chains to represent
 /// the gap between traces.
 const chainGap = '===== asynchronous gap ===========================\n';
 
 /// Returns [string] with enough spaces added to the end to make it [length]
 /// characters long.
 String padRight(String string, int length) {
   if (string.length >= length) return string;
 
   var result = new StringBuffer();
   result.write(string);
   for (var i = 0; i < length - string.length; i++) {
     result.write(' ');
   }
 
   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;
-}