support the JS builtin

test/codegen/expect/_foreign_helper/_foreign_helper.js

 var _foreign_helper;
 (function(_foreign_helper) {
   'use strict';
   // Function JS: (String, String, [dynamic, dynamic, dynamic, dynamic, dynamic, dynamic, dynamic, dynamic, dynamic, dynamic, dynamic, dynamic]) → dynamic
   function JS(typeDescription, codeTemplate, arg0, arg1, arg2, arg3, arg4, arg5, arg6, arg7, arg8, arg9, arg10, arg11) {
     if (arg0 === void 0)
       arg0 = null;
     if (arg1 === void 0)
       arg1 = null;
     if (arg2 === void 0)
@@ skipping 101 matching lines @@
   function JS_EFFECT(code) {
     dart.dinvokef(code, null);
   }
   class JS_CONST extends dart.Object {
     JS_CONST(code) {
       this.code = code;
     }
   }
   // Function JS_STRING_CONCAT: (String, String) → String
   function JS_STRING_CONCAT(a, b) {
-    return dart.as(JS('String', '# + #', a, b), core.String);
+    return a + b;
   }
   // Exports:
   _foreign_helper.JS = JS;
   _foreign_helper.JS_CURRENT_ISOLATE_CONTEXT = JS_CURRENT_ISOLATE_CONTEXT;
   _foreign_helper.IsolateContext = IsolateContext;
   _foreign_helper.JS_CALL_IN_ISOLATE = JS_CALL_IN_ISOLATE;
   _foreign_helper.DART_CLOSURE_TO_JS = DART_CLOSURE_TO_JS;
   _foreign_helper.RAW_DART_FUNCTION_REF = RAW_DART_FUNCTION_REF;
   _foreign_helper.JS_SET_CURRENT_ISOLATE = JS_SET_CURRENT_ISOLATE;
   _foreign_helper.JS_CREATE_ISOLATE = JS_CREATE_ISOLATE;
@@ skipping 14 matching lines @@
   _foreign_helper.JS_FUNCTION_TYPE_REQUIRED_PARAMETERS_TAG = JS_FUNCTION_TYPE_REQUIRED_PARAMETERS_TAG;
   _foreign_helper.JS_FUNCTION_TYPE_OPTIONAL_PARAMETERS_TAG = JS_FUNCTION_TYPE_OPTIONAL_PARAMETERS_TAG;
   _foreign_helper.JS_FUNCTION_TYPE_NAMED_PARAMETERS_TAG = JS_FUNCTION_TYPE_NAMED_PARAMETERS_TAG;
   _foreign_helper.JS_GET_NAME = JS_GET_NAME;
   _foreign_helper.JS_EMBEDDED_GLOBAL = JS_EMBEDDED_GLOBAL;
   _foreign_helper.JS_GET_FLAG = JS_GET_FLAG;
   _foreign_helper.JS_EFFECT = JS_EFFECT;
   _foreign_helper.JS_CONST = JS_CONST;
   _foreign_helper.JS_STRING_CONCAT = JS_STRING_CONCAT;
 })(_foreign_helper || (_foreign_helper = {}));