Use helper methods for running pub.

sdk/lib/_internal/pub/test/hosted/offline_test.dart

 // Copyright (c) 2012, 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 'dart:io';
 
 import '../descriptor.dart' as d;
 import '../test_pub.dart';
@@ skipping 32 matching lines @@
 
     integration('fails gracefully if a dependency is not cached', () {
       // Run the server so that we know what URL to use in the system cache.
       servePackages([]);
 
       d.appDir([
         dependencyMap("foo", "any")
       ]).create();
 
       pubCommand(command, args: ['--offline'],
-          error: new RegExp('Could not find package "foo" in cache'));
+          error: 'Could not find package "foo" in cache.');
     });
 
     integration('fails gracefully no cached versions match', () {
       // Run the server so that we know what URL to use in the system cache.
       servePackages([]);
 
       d.cacheDir({
         "foo": ["1.2.2", "1.2.3"]
       }, includePubspecs: true).create();
 
       d.appDir([
         dependencyMap("foo", ">2.0.0")
       ]).create();
 
-      pubCommand(command, args: ['--offline'],
-          error: new RegExp("Package 'foo' has no versions that match >2.0.0"));
+      pubCommand(command, args: ['--offline'], error:
+          "Package 'foo' has no versions that match >2.0.0 derived from:\n"
+          "- 'myapp' depends on version >2.0.0");
     });
   });
 }