Support `void` as generic argument.

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 fc1987ad7e412cd56933c16459bf4cf28ffb2bd6..cc0d415c389fa10977b950ceec5144a7cae7dbbe 100644
--- a/sdk/lib/_internal/js_runtime/lib/js_helper.dart
+++ b/sdk/lib/_internal/js_runtime/lib/js_helper.dart
@@ -183,6 +183,14 @@ bool isNullTypeRti(type) {
       JS_GET_NAME(JsGetName.NULL_CLASS_TYPE_NAME));
 }
 
+/// Returns whether the given type is void type.
+// TODO(floitsch): move this to foreign_helper.dart or similar.
+@ForceInline()
+bool isVoidType(rti) {
+  return JS_BUILTIN(
+      'returns:bool;effects:none;depends:none', JsBuiltin.isVoidRti, rti);
+}
+
 /// Returns the metadata of the given [index].
 // TODO(floitsch): move this to foreign_helper.dart or similar.
 @ForceInline()