Make pub strong-mode clean.

lib/src/barback/cycle_exception.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS d.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.
 
 import '../exceptions.dart';
 
 /// An exception thrown when a transformer dependency cycle is detected.
 ///
 /// A cycle exception is usually produced within a deeply-nested series of
 /// calls. The API is designed to make it easy for each of these calls to add to
 /// the message so that the full reasoning for the cycle is made visible to the
 /// user.
 ///
 /// Each call's individual message is called a "step". A [CycleException] is
 /// represented internally as a linked list of steps.
 class CycleException implements ApplicationException {
   /// The step for this exception.
   final String _step;
 
   /// The next exception in the linked list.
   ///
   /// [_next]'s steps come after [_step].
   final CycleException _next;
 
   /// A list of all steps in the cycle.
   List<String> get steps {
     if (_step == null) return [];
 
     var exception = this;
-    var steps = [];
+    var steps = <String>[];
     while (exception != null) {
       steps.add(exception._step);
       exception = exception._next;
     }
     return steps;
   }
 
   String get message {
     var steps = this.steps;
     if (steps.isEmpty) return "Transformer cycle detected.";
     return "Transformer cycle detected:\n" +
         steps.map((step) => "  $step").join("\n");
   }
 
   /// Creates a new [CycleException] with zero or one steps.
   CycleException([this._step])
       : _next = null;
 
   CycleException._(this._step, this._next);
 
   /// Returns a copy of [this] with [step] added to the beginning of [steps].
   CycleException prependStep(String step) {
     if (_step == null) return new CycleException(step);
     return new CycleException._(step, this);
   }
 
   String toString() => message;
 }