allow JS builtin to be typed as needed

lib/runtime/dart/js.js

 dart_library.library('dart/js', null, /* Imports */[
   'dart/_runtime',
   'dart/core',
   'dart/collection',
   'dart/_js_helper'
 ], /* Lazy imports */[
 ], function(exports, dart, core, collection, _js_helper) {
   'use strict';
   let dartx = dart.dartx;
   const _global = dart.global;
@@ skipping 372 matching lines @@
   dart.fn(allowInterop, core.Function, [core.Function]);
   dart.defineLazyProperties(exports, {
     get _interopCaptureThisExpando() {
       return new (core.Expando$(core.Function))();
     },
     set _interopCaptureThisExpando(_) {}
   });
   function allowInteropCaptureThis(f) {
     let ret = exports._interopCaptureThisExpando.get(f);
     if (ret == null) {
-      ret = dart.as(function() {
+      ret = function() {
         let args = [this];
         for (let arg of arguments) {
           args.push(arg);
         }
         return f(...args);
-      }, core.Function);
+      };
       exports._interopCaptureThisExpando.set(f, ret);
     }
     return ret;
   }
   dart.fn(allowInteropCaptureThis, core.Function, [core.Function]);
   const __CastType0$ = dart.generic(function(E) {
     const __CastType0 = dart.typedef('__CastType0', () => dart.functionType(dart.dynamic, [E]));
     return __CastType0;
   });
   let __CastType0 = __CastType0$();
   // Exports:
   exports.JsObject = JsObject;
   exports.JsFunction = JsFunction;
   exports.JsArray$ = JsArray$;
   exports.JsArray = JsArray;
   exports.allowInterop = allowInterop;
   exports.allowInteropCaptureThis = allowInteropCaptureThis;
 });