pkg/unittest: fix analyzer errors

test/backend/platform_selector/ast_test.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.
 
 import 'package:unittest/unittest.dart';
-import 'package:unittest/src/backend/platform_selector/ast.dart';
 import 'package:unittest/src/backend/platform_selector/parser.dart';
 
 void main() {
   group("toString() for", () {
     test("a variable is its name", () {
       _expectToString("foo");
       _expectToString("a-b");
     });
 
     group("not", () {
@@ skipping 57 matching lines @@
   });
 }
 
 void _expectToString(String selector, [String result]) {
   if (result == null) result = selector;
   expect(_toString(selector), equals(result),
       reason: 'Expected toString of "$selector" to be "$result".');
 }
 
 String _toString(String selector) => new Parser(selector).parse().toString();