Load patches via callback.

tests/compiler/dart2js/exit_code_test.dart

Index: tests/compiler/dart2js/exit_code_test.dart
diff --git a/tests/compiler/dart2js/exit_code_test.dart b/tests/compiler/dart2js/exit_code_test.dart
index bc4549e8305aa7b42e928ebe5f41f624cb68727f..77ccb3b44da8616eb9537bb0487f59dcec609ade 100644
--- a/tests/compiler/dart2js/exit_code_test.dart
+++ b/tests/compiler/dart2js/exit_code_test.dart
@@ -17,6 +17,7 @@ import 'package:compiler/implementation/dart2js.dart' as entry;
 import 'package:compiler/implementation/dart2jslib.dart';
 import 'package:compiler/implementation/apiimpl.dart' as apiimpl;
 import 'package:compiler/implementation/elements/elements.dart';
+import 'package:compiler/implementation/library_loader.dart';
 import 'package:compiler/implementation/resolution/resolution.dart';
 import 'package:compiler/implementation/scanner/scannerlib.dart';
 import 'package:compiler/implementation/util/util.dart';
@@ -48,9 +49,10 @@ class TestCompiler extends apiimpl.Compiler {
     return super.run(uri);
   }
 
-  void onLibraryScanned(LibraryElement element) {
+  Future onLibraryScanned(LibraryElement element,
+                        LibraryLoaderCallback callback) {
     test('Compiler.onLibraryScanned');
-    super.onLibraryScanned(element);
+    return super.onLibraryScanned(element, callback);
   }
 
   Future onLibrariesLoaded(Map<Uri, LibraryElement> loadedLibraries) {