Fix all strong mode warnings.

lib/src/evaluator.dart

Index: lib/src/evaluator.dart
diff --git a/lib/src/evaluator.dart b/lib/src/evaluator.dart
index 9102add652703809758b50a3a6d4aa382851639a..6401a374b6a3f561d45279e17413c95e4aa28604 100644
--- a/lib/src/evaluator.dart
+++ b/lib/src/evaluator.dart
@@ -2,6 +2,8 @@
 // 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 'package:collection/collection.dart';
+
 import 'ast.dart';
 import 'visitor.dart';
 
@@ -15,8 +17,8 @@ class Evaluator implements Visitor<bool> {
 
   Evaluator(semantics)
       : _semantics = semantics is Iterable
-            ? semantics.toSet().contains
-            : semantics;
+            ? DelegatingIterable.typed(semantics.toSet()).contains
+            : semantics as _Semantics;
 
   bool visitVariable(VariableNode node) => _semantics(node.name);