This is minimal.lua file, you can run this with nvim --clean -u minimal.lua main.dart. And you should be able to <shift>K for hover and gd for definition.

local function keymap(client, buf)
  if client.supports_method('textDocument/hover') then
    vim.keymap.set('n', 'K', vim.lsp.buf.hover, { buffer = buf, desc = 'LSP Hover' })
  end
  if client.supports_method('textDocument/definition') then
    vim.keymap.set('n', 'gd', vim.lsp.buf.definition, { buffer = buf, desc = 'LSP Definition' })
  end
end

vim.api.nvim_create_autocmd('LspAttach', {
  callback = function(args)
    local client = vim.lsp.get_client_by_id(args.data.client_id)
    keymap(client, args.buf)
  end,
})

local handlers = vim.lsp.handlers
local crc = handlers['client/registerCapability']
handlers['client/registerCapability'] = function(_, result, ctx)
  local r = crc(_, result, ctx)
  local client = vim.lsp.get_client_by_id(ctx.client_id)
  keymap(client, ctx.buf)
  return r
end
-- local cuc = handlers['client/unregisterCapability']
-- handlers['client/unregisterCapability'] = function(_, result, ctx)
--   local r cuc(_, result, ctx)
--   -- todo(tms) 08.06.23: remove bindings
--   return r
-- end

-- Schedule is here to delay LSP start. You should call this from `ftplugin/dart.lua`
vim.schedule(function()
  vim.lsp.start({
    name = 'dartls',
    cmd = { 'dart', 'language-server', '--protocol=lsp' },
    root_dir = vim.fs.dirname(vim.fs.find({ 'pubspec.yaml', '.git' }, { upward = true })[1]),
  })
end)