Add missing import.

test/dependency_computer/error_test.dart

 // Copyright (c) 2014, 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 pub_tests;
 
+import 'package:analyzer/analyzer.dart';
 import 'package:pub/src/exceptions.dart';
 import 'package:scheduled_test/scheduled_test.dart';
 
 import '../descriptor.dart' as d;
 import '../test_pub.dart';
 import 'utils.dart';
 
 void main() {
   initConfig();
 
@@ skipping 24 matching lines @@
         "transformers": ["myapp"]
       }),
       d.dir('lib', [
         d.file("myapp.dart", "library;")
       ])
     ]).create();
 
     expectException(new isInstanceOf<AnalyzerErrorGroup>());
   });
 }