Throw ArgumentError if packageRoot is null.

dart/tests/compiler/dart2js/mirrors_test.dart

Index: dart/tests/compiler/dart2js/mirrors_test.dart
diff --git a/dart/tests/compiler/dart2js/mirrors_test.dart b/dart/tests/compiler/dart2js/mirrors_test.dart
index 339eb3a4496514fcd7bd403a30b191b4fac5951d..2b5d9924dcf533d8c5f265a656ce7ab4cac35b95 100644
--- a/dart/tests/compiler/dart2js/mirrors_test.dart
+++ b/dart/tests/compiler/dart2js/mirrors_test.dart
@@ -43,13 +43,14 @@ DeclarationMirror findMirror(Iterable<DeclarationMirror> list, Symbol name) {
 
 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) {