Throw ArgumentError if packageRoot is null.

dart/tests/compiler/dart2js/mirrors_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.
 
 import "package:expect/expect.dart";
 import "package:async_helper/async_helper.dart";
 import 'package:compiler/src/mirrors/source_mirrors.dart';
 import 'package:compiler/src/mirrors/mirrors_util.dart';
 import 'package:compiler/src/mirrors/analyze.dart';
 import 'package:compiler/src/filenames.dart'
@@ skipping 25 matching lines @@
   for (DeclarationMirror mirror in list) {
     if (mirror.simpleName == name) {
       return mirror;
     }
   }
   return null;
 }
 
 main() {
   Uri scriptUri = currentDirectory.resolveUri(Platform.script);
+  Uri packageRoot = scriptUri.resolve('./packages/');
   Uri libUri = scriptUri.resolve('../../../sdk/');
   Uri inputUri = scriptUri.resolve('mirrors_helper.dart');
   var provider = new CompilerSourceFileProvider();
   var diagnosticHandler =
         new FormattingDiagnosticHandler(provider).diagnosticHandler;
   asyncStart();
-  var result = analyze([inputUri], libUri, null,
+  var result = analyze([inputUri], libUri, packageRoot,
                        provider.readStringFromUri, diagnosticHandler,
                        <String>['--preserve-comments']);
   result.then((MirrorSystem mirrors) {
     test(mirrors);
   }).then(asyncSuccess);
 }
 
 void test(MirrorSystem mirrors) {
   Expect.isNotNull(mirrors, "No mirror system returned from compilation");
 
@@ skipping 923 matching lines @@
   Expect.isTrue(privateFactoryConstructor.isPrivate);
   Expect.isFalse(privateFactoryConstructor.isConstConstructor);
   Expect.isFalse(privateFactoryConstructor.isRedirectingConstructor);
   Expect.isFalse(privateFactoryConstructor.isGenerativeConstructor);
   Expect.isTrue(privateFactoryConstructor.isFactoryConstructor);
 
   var metadata = privateClass.metadata;
   Expect.isNotNull(metadata);
   Expect.equals(0, metadata.length);
 }