Make is and as checks handle JavaScriptFunction correctly.

sdk/lib/_internal/js_runtime/lib/js_helper.dart

Index: sdk/lib/_internal/js_runtime/lib/js_helper.dart
diff --git a/sdk/lib/_internal/js_runtime/lib/js_helper.dart b/sdk/lib/_internal/js_runtime/lib/js_helper.dart
index e8b0a6bded4316b37da9f1b771d5c253647ef2c7..8b3020a56a7a93278921d19b403656b8bd3ec460 100644
--- a/sdk/lib/_internal/js_runtime/lib/js_helper.dart
+++ b/sdk/lib/_internal/js_runtime/lib/js_helper.dart
@@ -3179,8 +3179,9 @@ propertyTypeCast(value, property) {
  */
 interceptedTypeCheck(value, property) {
   if (value == null) return value;
-  if ((identical(JS('String', 'typeof #', value), 'object'))
-      && JS('bool', '#[#]', getInterceptor(value), property)) {
+  if ((JS('bool', 'typeof # === "object"', value) ||
+              JS('bool', 'typeof # === "function"', value)) &&
+      JS('bool', '#[#]', getInterceptor(value), property)) {
     return value;
   }
   propertyTypeError(value, property);
@@ -3192,9 +3193,10 @@ interceptedTypeCheck(value, property) {
  * prototype at load time.
  */
 interceptedTypeCast(value, property) {
-  if (value == null
-      || ((JS('bool', 'typeof # === "object"', value))
-          && JS('bool', '#[#]', getInterceptor(value), property))) {
+  if (value == null ||
+      ((JS('bool', 'typeof # === "object"', value) ||
+              JS('bool', 'typeof # === "function"', value)) &&
+          JS('bool', '#[#]', getInterceptor(value), property))) {
     return value;
   }
   propertyTypeCastError(value, property);