Fix all strong mode warnings.

lib/src/evaluator.dart

 // Copyright (c) 2016, 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.
 
+import 'package:collection/collection.dart';
+
 import 'ast.dart';
 import 'visitor.dart';
 
 typedef bool _Semantics(String variable);
 
 /// A visitor for evaluating boolean selectors against a specific set of
 /// semantics.
 class Evaluator implements Visitor<bool> {
   /// The semantics to evaluate against.
   final _Semantics _semantics;
 
   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);
 
   bool visitNot(NotNode node) => !node.child.accept(this);
 
   bool visitOr(OrNode node) =>
       node.left.accept(this) || node.right.accept(this);
 
   bool visitAnd(AndNode node) =>
       node.left.accept(this) && node.right.accept(this);
 
   bool visitConditional(ConditionalNode node) => node.condition.accept(this)
       ? node.whenTrue.accept(this)
       : node.whenFalse.accept(this);
 }