'Unknown instance kind: closure' when hovering over some functions

[annagrin]

The experimental expression evaluation feature supports evaluating closures, but causes an error displayed in VSCode debug console (see image below).

Repro steps

• Install flutter
• Use flutter's example/hello_world, set a breakpoint in
  flutter/packages/flutter/lib/src/widgets/binding.dart:921
• Add --web-enable-expression-evaluation flag to dart args in launch.json:

   {
       "version": "0.2.0",
       "configurations": [
           {
               "name": "Flutter",
               "request": "launch",
               "type": "dart",
               "args": ["--web-enable-expression-evaluation"],
           }
       ]
   }

• hover over attachRootWidget

Actual behavior

• native function text without a body is displayed in pop-up box, and appears to not have a lot of useful information
• Unknown instance kind: closure error appears in the debug console

Expected behavior

• some useful representation of the function displayed, for example:
  • maybe full text (if small)
  • or a signature and a few lines
  • or no popup at all
• no error in display window

[DanTup]

The reason for the printed warning is that we were trying to render this as a variable (but it has no fields, association, getters, etc.). I've fixed that by treating a Closure as a simple type.

The reason it shows "function [native code ]" is that we invoke toString() on values when displaying them. In the standard VM, this returns a slightly more useful string Closure: (Widget) => void from Function 'attachRootWidget':.

{
	"jsonrpc": "2.0",
	"result": {
		"type": "@Instance",
		"_vmType": "String",
		"class": {
			"type": "@Class",
			"fixedId": true,
			"id": "classes\/80",
			"name": "_OneByteString",
			"_vmName": "_OneByteString@0150898"
		},
		"kind": "String",
		"id": "objects\/100",
		"length": 60,
		"valueAsString": "Closure: (Widget) => void from Function 'attachRootWidget':."
	},
	"id": "372"
}

I think the best way to improve this would probably be by handling toString() in the proxy?

I'd probably prefer VS Code to fall back to the analysis server hover for things like this (since it has the dartdoc etc. in it), but that's not supported when in a debug session.

[annagrin]

@DanTup thanks for fixing this! I've created an issue on the debugger side to track better printing.