Change IllegalArgumentException to ArgumentError.

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 62898c790b51ab0c659e6dabe4cce67845ff1770..06fb46b9ba15ac258e9a8cc237b7e6cc0a13ef5c 100644
--- a/runtime/lib/expando_patch.dart
+++ b/runtime/lib/expando_patch.dart
@@ -58,7 +58,7 @@ patch class Expando<T> {
       throw new NullPointerException();
     }
     if (object is bool || object is num || object is String) {
-      throw new IllegalArgumentException(object);
+      throw new ArgumentError(object);
     }
   }