non nullable dataset [] operator

[fsw]

Issue:

In both JavaScript and dart:html the dataset property can be used to check if given data-* attribute is set.

For example in dart:

if (div.dataset['someData'] != null) {
  doSomethingWith(div.dataset['someData']);
}

In JS undefined attribute will return undefined.

However, currently in package:web, DOMStringMap's operator [] returns non-nullable String :

extension type DOMStringMap._(JSObject _) implements JSObject {
  external String operator [](String name);
}

Because of this, in package:web, code like if (div.dataset['someData'] != null) will generate an analyzer warning:

The operand can't be 'null', so the condition is always 'true'.

And if the data-some-data does not exist it will throw runtime error on execution:

TypeError: null: type 'JSNull' is not a subtype of type 'String'

So to achieve above logic in package:web I have to do something like:

if (div.hasAttribute('data-some-data')) {
  doSomethingWith(div.dataset['someData']);
}

Which makes dataset less useful and more ugly than dart:html version.
I could use getAttribute that returns String? to do the above but then the dataset becomes not useful at all.

Also in current implementation there is no way to delete elements from dataset (JS uses unnamed deleter and delete operator).

Proposed solution:

Simplest solution would be to replace 'String' with 'String?' in 'DOMStringMap' (tested this and it works fine with wasm and dart2js) but if i understand this correctly, this interface is generated from DOMStringMap IDL and it seem to match the definition and the issue is we do not have 'undefined'/'null' distinction in Dart?

So instead i propose a solution to add data wrapper on dataset that will handle this issue and allow checking for attribute:

if (div.data['someData'] != null) {}

This wrapper can also add 'remove' method.

I will submit a PR with above in a minute, I am creating this issue to discuss if maybe there is a better solution (maybe modifying the generation process to consider cases like this?).

[ykmnkmi]

From the specs used to generate most extensions:

interface DOMStringMap {
  getter DOMString (DOMString name);
  setter undefined (DOMString name, DOMString value);
  deleter undefined (DOMString name);
}

PS: I saw your PR.