Fix analysis warnings.

lib/src/backend/platform_selector/evaluator.dart

 // Copyright (c) 2015, 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.
 
 library unittest.backend.platform_selector.evaluator;
 
-import 'package:source_span/source_span.dart';
-
 import '../operating_system.dart';
 import '../test_platform.dart';
 import 'ast.dart';
 import 'visitor.dart';
 
 /// A visitor for evaluating platform selectors against a specific
 /// [TestPlatform] and [OperatingSystem].
 class Evaluator implements Visitor<bool> {
   /// The platform to test against.
   final TestPlatform _platform;
@@ skipping 23 matching lines @@
   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);
 }