void setupJsTypeCache()

Source 

void setupJsTypeCache() {
  // Cache the @JS Type.
  if (_atJsType == -1) {
    var uri = new Uri(scheme: "package", path: "js/js.dart");
    var jsLibrary = mirrors.currentMirrorSystem().libraries[uri];
    if (jsLibrary != null) {
      // @ JS used somewhere.
      var jsDeclaration = jsLibrary.declarations[new Symbol("JS")];
      _atJsType = jsDeclaration.reflectedType;
    } else {
      // @ JS not used in any library.
      _atJsType = null;
    }
  }
}