Support automatically displaying signature information, when appropriate

The new Signature Help feature triggers almost constantly, even when it doesn’t make sense to, like inside of comments. Or the signature shown will be for something the next layer up.

Example code:

return ListTile(
  title: Text("Enable notifications"),
  trailing: Switch(
    value: notificationsEnabled,
    onChanged: (bool enabling){
      if (enabling) {
        // type this comma -> , and get the signature of Switch()
      } else {
        // ...
      }
    },
  ),
);

[DanTup]

Fixed in https://github.com/dart-lang/sdk/commit/cef054ae311cebbf1b9efb5d15b2ad1fc36f8d54.

[DanTup]

Working on this in the LSP server at: https://dart-review.googlesource.com/c/sdk/+/162000

If the request for signature info is from an automatic trigger when typing ( then we’ll only return results if the ( is exactly the offset of the located ArgumentList. Comma is now just a “retrigger” character, so it will only ever trigger signature help if it’s already visible (and therefore returning results will just update the UI and not trigger any new popups).