Prevent extra method.toString checks.

It appears that the intent here was to only check the
`method.toString().indexOf('_super')` once, and store the result so that
we do not need to check the same method twice, but we were never actual
storing `__hasSuper` on the method so it was just always going to check
the `toString`.
Merge pull request #5480 from rwjblue/prevent-extra-toStrings

Prevent extra method.toString checks.
String.prototype.indexOf returns -1 for non-matches.

Previous check would wrap everything (unless `_super` was at position
`0` which is not possible).
Merge pull request #5481 from rwjblue/properly-use-indexOf

String.prototype.indexOf returns -1 for non-matches.
[DOC release] Update Ember.computed with depKeys and example
Merge pull request #5486 from knownasilya/docs-computed-dep-keys

[DOC release] Update Ember.computed with depKeys and example
[Bugfix beta] ensure pushUnique targetQueue is cleared by flush
Merge pull request #5490 from stefanpenner/fix-bb

[Bugfix beta] ensure pushUnique targetQueue is cleared by flush
remove unused vars and enforce in jshint