Fix checkboxes, move tests to input tests
Move argument special casing to the helpers.

Paves the way for Handlbars compat version of `registerHelper` and
`makeBoundHelper`. Also, cleans up troubling bits of `streamifyArgs`.
Merge pull request #9627 from rwjblue/helpers-preprocessArguments

Move argument special casing to the helpers.
Merge pull request #9626 from mixonic/checkbox

Fix checkboxes, move tests to input tests
Add several tests to HTMLBars feature flag
Merge pull request #9629 from mixonic/remove-flags

Add tests to HTMLBars feature flag
Add HTMLBars `makeHandlebarsCompatibleHelper` function.
[DOC release] Fix doc bug where forEach parameter order is wrong

Seems the code was updated to match ES6 but not the docs.
Merge pull request #9631 from rwjblue/htmlbars-handlebars-compat

Add HTMLBars `makeHandlebarsCompatibleHelper` function.
Merge pull request #9632 from thomassnielsen/patch-2

[DOC release] Fix doc bug where forEach parameter order is wrong
[BUGFIX beta] Ember.Map#forEach callback should include map being iterated over

According to the ES6-spec, the callback for `forEach` on `Map` should include the Map being iterated over as the third parameter.

Source: https://people.mozilla.org/~jorendorff/es6-draft.html#sec-map.prototype.foreach
Merge pull request #9633 from kimroen/bugfix-ember-map-foreach

[BUGFIX beta] Ember.Map#forEach callback should include map being ite...
Fix copy-paste error in registerAsyncHelper doc example
Merge pull request #9636 from balinterdi/fix-testing-helper-doc-example

Fix copy-paste error in registerAsyncHelper doc example
Make HTMLBars helpers instances of a Helper object.
Merge pull request #9638 from rwjblue/make-helpers-an-object

Make HTMLBars helpers instances of a Helper object.
Add HTMLBars version of `makeBoundHelper`.
Update HTMLBars helpers to use their prototype better.
Merge pull request #9639 from rwjblue/htmlbars-makeBoundHelper

Add HTMLBars version of `makeBoundHelper`.
Merge pull request #9640 from rwjblue/better-helper-object-shape

Update HTMLBars helpers to use their prototype better.