html stubs (should be discussed)
(cherry picked from commit ecb334b)
better js:
var tmp$4;
list.add((tmp$4 = errorCallback) !== null && tmp$4 !== undefined ? tmp$4 : Kotlin.throwNPE());

can be just

list.add(errorCallback === null || errorCallback === undefined ? errorCallback : Kotlin.throwNPE());
#KT-2670 Fixed
(cherry picked from commit 84826ef)