Update dart analysis_server command (#4761)

* Update dart analysis_server command

In 2021 the dart team added a new sub-command `language-server` to
replace the original `./snapshots/analysis_server.dart.snapshot --lsp`
convention for starting the language server.

dart-lang/sdk@c224cc2

* Add ale_dart_analysis_server_enable_language_server option

This allows users to opt-in to the new `dart language-server` command.

* Enable ale_dart_analysis_server_enable_language_server option by default

* Update doc/ale-dart.txt

Include the dart version number where the `dart language-server` command
was added.

ale_linters/dart/analysis_server.vim

@@ -1,6 +1,7 @@
 " Author: Nelson Yeung <[email protected]>
 " Description: Check Dart files with dart analysis server LSP
 
+call ale#Set('dart_analysis_server_enable_language_server', 1)
 call ale#Set('dart_analysis_server_executable', 'dart')
 
 function! ale_linters#dart#analysis_server#GetProjectRoot(buffer) abort
@@ -12,12 +13,19 @@ function! ale_linters#dart#analysis_server#GetProjectRoot(buffer) abort
 endfunction
 
 function! ale_linters#dart#analysis_server#GetCommand(buffer) abort
+    let l:language_server = ale#Var(a:buffer, 'dart_analysis_server_enable_language_server')
     let l:executable = ale#Var(a:buffer, 'dart_analysis_server_executable')
     let l:dart = resolve(exepath(l:executable))
-
-    return '%e '
+    let l:output = '%e '
     \   . fnamemodify(l:dart, ':h') . '/snapshots/analysis_server.dart.snapshot'
     \   . ' --lsp'
+
+    " Enable new language-server command
+    if l:language_server == 1
+        let l:output = '%e language-server --protocol=lsp'
+    endif
+
+    return l:output
 endfunction
 
 call ale#linter#Define('dart', {

doc/ale-dart.txt

@@ -27,6 +27,19 @@ g:ale_dart_analysis_server_executable   *g:ale_dart_analysis_server_executable*
   This variable can be set to change the path of dart.
 
 
+g:ale_dart_analysis_server_enable_language_server
+                            *g:ale_dart_analysis_server_enable_language_server*
+                            *b:ale_dart_analysis_server_enable_language_server*
+  Type: |Number|
+  Default: `1`
+
+  When set to `1`, ALE will use the new `dart language-server` command,
+  available from Dart version 2.16.0, to launch the language server. When set
+  to `0`, ALE will instead use the deprecated
+  `./snapshots/analysis_server.dart.snapshot --lsp` command used by older
+  versions of Dart.
+
+
 ===============================================================================
 dart-analyze                                                 *ale-dart-analyze*
 

test/linter/test_dart_analysis_server.vader

@@ -6,10 +6,16 @@ After:
 
 Execute(The default command should be correct):
   AssertLinter 'dart', ale#Escape('dart')
-  \   . ' ./snapshots/analysis_server.dart.snapshot --lsp'
+  \   . ' language-server --protocol=lsp'
 
 Execute(The executable should be configurable):
   let g:ale_dart_analysis_server_executable = 'foobar'
 
   AssertLinter 'foobar', ale#Escape('foobar')
+  \   . ' language-server --protocol=lsp'
+
+Execute(Should be able to disable new language-server command):
+  let g:ale_dart_analysis_server_enable_language_server = 0
+
+  AssertLinter 'dart', ale#Escape('dart')
   \   . ' ./snapshots/analysis_server.dart.snapshot --lsp'