Allow tests to specify a package root.

dart/pkg/args/test/args_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 args_test;
 
 import '../../../pkg/unittest/lib/unittest.dart';
 
-// TODO(rnystrom): Use "package:" URL here when test.dart can handle pub.
-import '../lib/args.dart';
+import 'package:args/args.dart';
 
 main() {
   group('ArgParser.addFlag()', () {
     test('throws ArgumentError if the flag already exists', () {
       var parser = new ArgParser();
       parser.addFlag('foo');
       throwsIllegalArg(() => parser.addFlag('foo'));
     });
 
     test('throws ArgumentError if the option already exists', () {
@@ skipping 682 matching lines @@
         throw new ArgumentError(
             'Line "$line" does not have enough indentation.');
       }
 
       lines[i] = line.substring(indent);
     }
   }
 
   return Strings.join(lines, '\n');
 }