allow JS builtin to be typed as needed

lib/runtime/dart/js.js

Index: lib/runtime/dart/js.js
diff --git a/lib/runtime/dart/js.js b/lib/runtime/dart/js.js
index de81b79fc2b8464b56030b5a7948c34bee836c3f..412f7803d8618e767342fe23eb166824d87a204c 100644
--- a/lib/runtime/dart/js.js
+++ b/lib/runtime/dart/js.js
@@ -390,13 +390,13 @@ dart_library.library('dart/js', null, /* Imports */[
   function allowInteropCaptureThis(f) {
     let ret = exports._interopCaptureThisExpando.get(f);
     if (ret == null) {
-      ret = dart.as(function() {
+      ret = function() {
         let args = [this];
         for (let arg of arguments) {
           args.push(arg);
         }
         return f(...args);
-      }, core.Function);
+      };
       exports._interopCaptureThisExpando.set(f, ret);
     }
     return ret;