NullableUndefineableJSAnyExtension extension

Helper methods to determine if a value is JavaScript undefined or null.

JavaScript undefined and JavaScript null are internalized differently based on the backend. When compiling to JavaScript, Dart null can actually be JavaScript undefined or JavaScript null. When compiling to Wasm, that's not the case: there's only one Wasm value null can be. Therefore, when an interop API returns JavaScript null or JavaScript undefined, they are both converted to Dart null when compiling to Wasm, and when you pass a Dart null to an interop API, it is called with JavaScript null. When compiling to JavaScript, Dart null retains its original JavaScript value. Avoid writing code where this distinction between null and undefined matters.