Incorrect distribution after adding a fragment that does not include an insertion point after adding a fragment that does

[sorvell]

Document fragments added to shadyRoots are scanned for insertion points. If one is found then the shadyRoot's list of insertion points is invalidated and on next distribution, the list is updated. However, if a fragment that does not include an insertion point is added after one that does but before distribution, the insertion point list invalidation improperly gets unset.

Problem spot: https://github.com/Polymer/polymer/blob/master/src/lib/dom-api-shady.html#L94