restore FutureOr cast failure ignore

pkg/dev_compiler/lib/js/es6/dart_sdk.js

Index: pkg/dev_compiler/lib/js/es6/dart_sdk.js
diff --git a/pkg/dev_compiler/lib/js/es6/dart_sdk.js b/pkg/dev_compiler/lib/js/es6/dart_sdk.js
index 49b08e6b633560208e092fabf557e5e4a5c37179..f24643c82608e859b1ac4e07d9ed5617ca0eca7f 100644
--- a/pkg/dev_compiler/lib/js/es6/dart_sdk.js
+++ b/pkg/dev_compiler/lib/js/es6/dart_sdk.js
@@ -20413,11 +20413,11 @@ async.FutureOr$ = dart.generic(T => {
   };
   FutureOr.as = function as_FutureOr(o) {
     if (o == null || T.is(o) || FutureOfT().is(o)) return o;
-    return dart.castError(o, this, false);
+    return dart.as(o, this, false);
   };
   FutureOr._check = function check_FutureOr(o) {
     if (o == null || T.is(o) || FutureOfT().is(o)) return o;
-    return dart.castError(o, this, true);
+    return dart.as(o, this, true);
   };
   return FutureOr;
 });