Return String should be nullable, no?

[kevmoo]

web/web/lib/src/dom/html.dart

Line 1281 in 6538aca

external String operator [](String name);

CC @srujzs

[srujzs]

This is similar to #181. The IDL claims the return type is non-nullable, but naturally it would be nice to coerce missing values to null instead of a cast failure. I'm not sure if the behavior is similar everywhere a getter is declared in the IDL to make that change. The current solution is using has to check for existence.