Throw ArgumentError if packageRoot is null.

dart/tests/compiler/dart2js/missing_file_test.dart

Index: dart/tests/compiler/dart2js/missing_file_test.dart
diff --git a/dart/tests/compiler/dart2js/missing_file_test.dart b/dart/tests/compiler/dart2js/missing_file_test.dart
index 69b76da479c4b239df080acafc2e7d2edf322279..92c94d54f395e142461e2b065d600d779db74364 100644
--- a/dart/tests/compiler/dart2js/missing_file_test.dart
+++ b/dart/tests/compiler/dart2js/missing_file_test.dart
@@ -30,6 +30,7 @@ main() {}
 void runCompiler(Uri main, String expectedMessage) {
   Uri script = currentDirectory.resolveUri(Platform.script);
   Uri libraryRoot = script.resolve('../../../sdk/');
+  Uri packageRoot = script.resolve('./packages/');
 
   var provider = new MemorySourceFileProvider(MEMORY_SOURCE_FILES);
   var handler = new FormattingDiagnosticHandler(provider);
@@ -53,7 +54,7 @@ void runCompiler(Uri main, String expectedMessage) {
                                    outputProvider,
                                    diagnosticHandler,
                                    libraryRoot,
-                                   null,
+                                   packageRoot,
                                    [],
                                    {});