Fix null.runtimeType, reflect(null).type.superinterfaces, and reflect(null).getField.

sdk/lib/_internal/lib/js_helper.dart

Index: sdk/lib/_internal/lib/js_helper.dart
diff --git a/sdk/lib/_internal/lib/js_helper.dart b/sdk/lib/_internal/lib/js_helper.dart
index 0d208f0033c81db5bb7af960a8b9475e46c5c4e8..cef540b64faed493536f0266130bc1057e14c6d0 100644
--- a/sdk/lib/_internal/lib/js_helper.dart
+++ b/sdk/lib/_internal/lib/js_helper.dart
@@ -978,6 +978,7 @@ class Primitives {
     if (JS('bool', '# == "num"', className)) return const JSNumber();
     if (JS('bool', '# == "bool"', className)) return const JSBool();
     if (JS('bool', '# == "List"', className)) return const JSArray();
+    if (JS('bool', '# == "Null"', className)) return const JSNull();
     return JS('var', 'init.allClasses[#]', className);
   }