Use helper methods for running pub.

sdk/lib/_internal/pub/test/unknown_source_test.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS d.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 d.file.
 
 library pub_tests;
 
 import 'dart:io';
 import 'dart:json' as json;
 
 import 'descriptor.dart' as d;
 import 'test_pub.dart';
 
 main() {
   initConfig();
 
   forBothPubInstallAndUpdate((command) {
     integration('fails gracefully on a dependency from an unknown source', () {
       d.appDir([{"bad": "foo"}]).create();
 
-      pubCommand(command, error: new RegExp(
-          "Package 'myapp' depends on 'foo' from unknown source 'bad'.\$"));
+      pubCommand(command, error:
+          "Package 'myapp' depends on 'foo' from unknown source 'bad'.");
     });
 
     integration('fails gracefully on transitive dependency from an unknown '
                 'source', () {
       d.dir('foo', [
         d.libDir('foo', 'foo 0.0.1'),
         d.libPubspec('foo', '0.0.1', deps: [{"bad": "bar"}])
       ]).create();
 
       d.appDir([{"path": "../foo"}]).create();
 
-      pubCommand(command, error: new RegExp(
-          "Package 'foo' depends on 'bar' from unknown source 'bad'.\$"));
+      pubCommand(command, error:
+          "Package 'foo' depends on 'bar' from unknown source 'bad'.");
     });
 
     integration('ignores unknown source in lockfile', () {
       d.dir('foo', [
         d.libDir('foo'),
         d.libPubspec('foo', '0.0.1')
       ]).create();
 
       // Depend on "foo" from a valid source.
       d.dir(appPath, [
@@ skipping 24 matching lines @@
 
       // Should update to the new one.
       d.dir(packagesPath, [
         d.dir("foo", [
           d.file("foo.dart", 'main() => "foo";')
         ])
       ]).validate();
     });
   });
 }