Support checked mode

pkg/compiler/lib/src/js_backend/runtime_types.dart

Index: pkg/compiler/lib/src/js_backend/runtime_types.dart
diff --git a/pkg/compiler/lib/src/js_backend/runtime_types.dart b/pkg/compiler/lib/src/js_backend/runtime_types.dart
index 369cec030ffccaeb63f61330dd7bb67e8c4590ee..570ea1c9ba382418e5a56cf750b3d827ac5baa1a 100644
--- a/pkg/compiler/lib/src/js_backend/runtime_types.dart
+++ b/pkg/compiler/lib/src/js_backend/runtime_types.dart
@@ -150,7 +150,7 @@ class _RuntimeTypes implements RuntimeTypes {
   @override
   void registerRtiDependency(Element element, Element dependency) {
     // We're not dealing with typedef for now.
-    if (!element.isClass || !dependency.isClass) return;
+    if (element == null || !element.isClass || !dependency.isClass) return;
     Set<ClassElement> classes =
         rtiDependencies.putIfAbsent(element, () => new Set<ClassElement>());
     classes.add(dependency);