Simplify the code for most linters and tests with closures

ale_linters/dart/dartanalyzer.vim

@@ -3,15 +3,10 @@
 
 call ale#Set('dart_dartanalyzer_executable', 'dartanalyzer')
 
-function! ale_linters#dart#dartanalyzer#GetExecutable(buffer) abort
-    return ale#Var(a:buffer, 'dart_dartanalyzer_executable')
-endfunction
-
 function! ale_linters#dart#dartanalyzer#GetCommand(buffer) abort
-    let l:executable = ale_linters#dart#dartanalyzer#GetExecutable(a:buffer)
     let l:path = ale#path#FindNearestFile(a:buffer, '.packages')
 
-    return ale#Escape(l:executable)
+    return '%e'
     \   . (!empty(l:path) ? ' --packages ' . ale#Escape(l:path) : '')
     \   . ' %s'
 endfunction
@@ -34,7 +29,7 @@ endfunction
 
 call ale#linter#Define('dart', {
 \   'name': 'dartanalyzer',
-\   'executable_callback': 'ale_linters#dart#dartanalyzer#GetExecutable',
+\   'executable_callback': ale#VarFunc('dart_dartanalyzer_executable'),
 \   'command_callback': 'ale_linters#dart#dartanalyzer#GetCommand',
 \   'callback': 'ale_linters#dart#dartanalyzer#Handle',
 \   'lint_file': 1,

ale_linters/dart/language_server.vim

@@ -3,10 +3,6 @@
 
 call ale#Set('dart_language_server_executable', 'dart_language_server')
 
-function! ale_linters#dart#language_server#GetExecutable(buffer) abort
-    return ale#Var(a:buffer, 'dart_language_server_executable')
-endfunction
-
 function! ale_linters#dart#language_server#GetProjectRoot(buffer) abort
     " Note: pub only looks for pubspec.yaml, there's no point in adding
     " support for pubspec.yml
@@ -18,7 +14,7 @@ endfunction
 call ale#linter#Define('dart', {
 \   'name': 'language_server',
 \   'lsp': 'stdio',
-\   'executable_callback': 'ale_linters#dart#language_server#GetExecutable',
+\   'executable_callback': ale#VarFunc('dart_language_server_executable'),
 \   'command_callback': 'ale_linters#dart#language_server#GetExecutable',
 \   'project_root_callback': 'ale_linters#dart#language_server#GetProjectRoot',
 \})