Make it easier to run engine tests

pkg/analyzer/test/error_test.dart

 // Copyright (c) 2013, 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 test.error;
+
 import 'package:unittest/unittest.dart';
 
 import 'utils.dart';
 
 void main() {
   test("a valid Dart file doesn't throw any errors", () {
     expect(
         errorsForFile('void main() => print("Hello, world!");'),
         isNull);
   });
@@ skipping 29 matching lines @@
   test("an error at the beginning of a very long line", () {
     expect(errorsForFile('void foo;     $veryLongString'),
         equals("Error in test.dart: Variables cannot have a type of 'void'\n"));
   });
 
   test("an error in the middle of a very long line", () {
     expect(errorsForFile('$veryLongString void foo;$veryLongString'),
         equals("Error in test.dart: Variables cannot have a type of 'void'\n"));
   });
 }