Make `NullRejectionException` constructor private

[srujzs]

It currently has a public constructor but I don't see a good reason to allow users to construct this exception themselves:

sdk/sdk/lib/js_util/js_util.dart

Line 141 in f0b83bf

NullRejectionException(this.isUndefined);

.

[sgrekhov]

Please note that isUndefined always returns false on Wasm. It probably makes sense to update the documentation to reflect this fact as part of this issue.

UPD. The fact that null and undefined are always treated as null on Wasm is documented in NullableUndefineableJSAnyExtension, which is not an obvious place. Users who haven’t read the NullableUndefineableJSAnyExtension documentation may expect isUndefined to return true on Wasm. That's why I think that it makes sense to add it here.