Void is not required to be `null` anymore.

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 d1866168b4859d69534c791d6f38d6bb8856f8ce..fc1987ad7e412cd56933c16459bf4cf28ffb2bd6 100644
--- a/sdk/lib/_internal/js_runtime/lib/js_helper.dart
+++ b/sdk/lib/_internal/js_runtime/lib/js_helper.dart
@@ -3416,11 +3416,6 @@ listSuperNativeTypeCast(value, property) {
   propertyTypeCastError(value, property);
 }
 
-voidTypeCheck(value) {
-  if (value == null) return value;
-  throw new TypeErrorImplementation(value, 'void');
-}
-
 extractFunctionTypeObjectFrom(o) {
   var interceptor = getInterceptor(o);
   var signatureName = JS_GET_NAME(JsGetName.SIGNATURE_NAME);