Revert "Remove NullPointerException."

runtime/lib/expando_patch.dart

Index: runtime/lib/expando_patch.dart
diff --git a/runtime/lib/expando_patch.dart b/runtime/lib/expando_patch.dart
index 5c619695bfb26a9de471c3536f313792d90b0ba6..a218af63b7ac57578910afaec5ee06c6548a65bd 100644
--- a/runtime/lib/expando_patch.dart
+++ b/runtime/lib/expando_patch.dart
@@ -54,7 +54,10 @@ patch class Expando<T> {
   }
 
   static _checkType(object) {
-    if (object == null || object is bool || object is num || object is String) {
+    if (object == null) {
+      throw new NullPointerException();
+    }
+    if (object is bool || object is num || object is String) {
       throw new ArgumentError(object);
     }
   }