helper(:all) raises error in an ActionView::TestCase

# test/helpers/gadgets_helper_test.rb
require 'test_helper'

class GadgetsHelperTest < ActionView::TestCase
  helper :all

  test "something" do
    # ...
  end
end

Running this yields the following error:

rails/activesupport/lib/active_support/dependencies.rb:277:in `depend_on': Missing helper file helpers/all_helper.rb (LoadError)

Here's the full backtrace:

$ rake test:units
(in /Users/dchelimsky/projects/ruby/rspec2/repos/rspec-rails/tmp/example_app)
/Users/dchelimsky/projects/ruby/rspec2/repos/rspec-rails/tmp/rails/activesupport/lib/active_support/dependencies.rb:277:in `depend_on': Missing helper file helpers/all_helper.rb (LoadError)
    from /Users/dchelimsky/projects/ruby/rspec2/repos/rspec-rails/tmp/rails/activesupport/lib/active_support/dependencies.rb:189:in `require_dependency'
    from /Users/dchelimsky/projects/ruby/rspec2/repos/rspec-rails/tmp/rails/actionpack/lib/abstract_controller/helpers.rb:131:in `modules_for_helpers'
    from /Users/dchelimsky/projects/ruby/rspec2/repos/rspec-rails/tmp/rails/actionpack/lib/abstract_controller/helpers.rb:127:in `map!'
    from /Users/dchelimsky/projects/ruby/rspec2/repos/rspec-rails/tmp/rails/actionpack/lib/abstract_controller/helpers.rb:127:in `modules_for_helpers'
    from /Users/dchelimsky/projects/ruby/rspec2/repos/rspec-rails/tmp/rails/actionpack/lib/abstract_controller/helpers.rb:91:in `helper'
    from ./test/unit/helpers/gadgets_helper_test.rb:4

helper(:all) works in controllers because controllers include ActionController::Metal::Helpers, but ActionView::TestCase::Behavior does not do the same.

I started to look at patching ActionView::TestCase::Behavior to include ActionController::Metal::Helpers, and got this very interesting error:

$ rake test:units
(in /Users/dchelimsky/projects/ruby/rspec2/repos/rspec-rails/tmp/example_app)
/Users/dchelimsky/projects/ruby/rspec2/repos/rspec-rails/tmp/rails/activesupport/lib/active_support/dependencies.rb:443:in `load_missing_constant': ActionController is not missing constant Helpers! (ArgumentError)

I'll continue to look at this, but I wanted to get this documented in the mean time.