Skip type checks on native JavaScriptObjects

lib/runtime/_types.js

Index: lib/runtime/_types.js
diff --git a/lib/runtime/_types.js b/lib/runtime/_types.js
index 482adf4fa1c714f73f63168c77b1fc3052d142ec..691d9f2e9739451e1ff4b51f431532f8a0c3f838 100644
--- a/lib/runtime/_types.js
+++ b/lib/runtime/_types.js
@@ -68,6 +68,12 @@ dart_library.library('dart_runtime/_types', null, /* Imports */[
   let bottomR = new Bottom();
   exports.bottom = bottomR;
 
+  class JSObject extends TypeRep {
+    toString() { return "NativeJavaScriptObject"; }
+  };
+  let jsobjectR = new JSObject();
+  exports.jsobject = jsobjectR;
+
   class AbstractFunctionType extends TypeRep {
     constructor() {
       super();