Lazily throw pubspec parsing exceptions.

sdk/lib/_internal/pub/test/install/path/no_pubspec_test.dart

 // Copyright (c) 2013, 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.
 
 import 'package:path/path.dart' as path;
 
 import '../../descriptor.dart' as d;
 import '../../test_pub.dart';
 
 main() {
   initConfig();
   integration('path dependency to non-package directory', () {
     // Make an empty directory.
     d.dir('foo').create();
     var fooPath = path.join(sandboxDir, "foo");
 
     d.dir(appPath, [
       d.appPubspec({
         "foo": {"path": fooPath}
       })
     ]).create();
 
-    pubInstall(error: 'Package "foo" doesn\'t have a pubspec.yaml file.');
+    pubInstall(error: new RegExp(r'Could not find a file named "pubspec.yaml" '
+        r'in "[^\n]*"\.'));
   });
 }