restore FutureOr cast failure ignore

pkg/dev_compiler/lib/js/amd/dart_sdk.js

Index: pkg/dev_compiler/lib/js/amd/dart_sdk.js
diff --git a/pkg/dev_compiler/lib/js/amd/dart_sdk.js b/pkg/dev_compiler/lib/js/amd/dart_sdk.js
index 2a8dc2ea27af23dec4d25af16a36cb64a18aa621..6218cbe46aafe2e0c808a5e23dd3eace388deb7d 100644
--- a/pkg/dev_compiler/lib/js/amd/dart_sdk.js
+++ b/pkg/dev_compiler/lib/js/amd/dart_sdk.js
@@ -20415,11 +20415,11 @@ define([], function() {
     };
     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;
   });