Add dart LSP (resolves #1006)

ale_linters/dart/language_server.vim

@@ -0,0 +1,30 @@
+" Author: aurieh <me@aurieh.me>
+" Description: A language server for dart
+
+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#GetLanguage(buffer) abort
+    return 'dart'
+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
+    let l:pubspec = ale#path#FindNearestFile(a:buffer, 'pubspec.yaml')
+
+    return !empty(l:pubspec) ? fnamemodify(l:pubspec, ':h:h') : ''
+endfunction
+
+call ale#linter#Define('dart', {
+\   'name': 'language_server',
+\   'lsp': 'stdio',
+\   'executable_callback': 'ale_linters#dart#language_server#GetExecutable',
+\   'command_callback': 'ale_linters#dart#language_server#GetExecutable',
+\   'language_callback': 'ale_linters#dart#language_server#GetLanguage',
+\   'project_root_callback': 'ale_linters#dart#language_server#GetProjectRoot',
+\})
+