Added isHTMLSafe to the ember-string shims.

[workmanw]

So Ember.String.isHTMLSafe was added in Ember 2.8. I was tempted to include the ember-string-ishtmlsafe-polyfill as a dependency, however we've been following the pattern of ember-assign-polyfill and ember-getowner-polyfill and neither of them are deps here.

I think it's logical to say that a consumer of ember-cli-shims using < Ember-2.8 has to also include the ember-string-ishtmlsafe-polyfill for themselves.

[locks]

Hey @workmanw, it is my understanding that the old shims are frozen and we intend to deprecate them as you can see in #92. I am closing this due to the previous reasoning.
From your description it sounds like you have a path forward? Ping on Slack if otherwise, I volunteer rwj ;)