Add SignatureHelpProvider to provide help for method/constructor args

[DanTup]

https://code.visualstudio.com/docs/extensionAPI/vscode-api#SignatureHelpProvider

[DanTup]

This isn't possible without a new feature in the Analyzer:

https://groups.google.com/a/dartlang.org/forum/#!topic/analyzer-discuss/Abh1vi1CwDA

[DanTup]

@devoncarew Do you have this functionality in Atom? If so, how to do you do it?

This is the biggest remaining annoyance for me when trying to write Dart in Dart Code. I've been trying to use Hovers to get the info but failing (it's hard to know where to issue the Hover command without understanding the surrounding Dart).

[devoncarew]

We don't support something like this. I don't know what the best options are. I think it would be a while until support for this comes to the analysis server. Two practical solutions might be to live w/o it, or to see if a heuristic w/ hovers can return info for many simple cases?

[DanTup]

I've started trying to use hover, but held off putting too much time into it without knowing if it'd be done in the analysis server. Brian confirmed it's likely to be a while, so I'll get back to it. I think the complications are:

1. The hover info is unstructured (just a string for all params)
2. Need to know where to send the hover request for

I think 1 will be easy to handle except in the case where there are arguments that are themselves functions. I think 2 is probably just a case of walking back to the first paren that has an alphanumeric before it (need to check if whitespace is allowed there).

The idea of forking the AS and building our own snapshot crossed my mind too, but I've not looked at the code at all to see how easily I could implement it myself using the existing stuff on the server side. Probably would be hard for me, but always an option.

I'll play with the easier options first anyway and see where I get. Now we have args in the completion info it's a little better than it was; but I'll have to sit down to try and write something to get a better idea of how much I'll miss this.

[DanTup]

Haven't gotten anywhere trying to use the hover stuff yet (figuring out the correct offset to send is non-trivial), so bumping this to 0.11.