Fix mocha-context-to-closure error when mixing setup/this/CallExpression

[GoodForOneFare]

esify spits out this error:

undefined does not match field "name": string of type Identifier
  test/theme-editor-next-javascripts/functional/design_mode/shopify_api_next_test.coffee

The problem is in this setup code:

    for key, event of DesignMode.API.EVENTS
      @[key.toLowerCase()] = @spy()
      document.removeEventListener(event, @[key.toLowerCase()])

Things are fine if:

• remove-useless-return-from-test is disabled
• @[key.toLowerCase()] lines are changed to foo[key.toLowerCase()]
• The toLowerCase() call is removed

Minimal failing test input:

suite('DesignMode.ShopifyApi', function() {
  setup(function() {
    for (let [key, event] of foo) {
      this[key.toLowerCase()] = bar;
    }
  });
});