Add clearer invariants to composite spec mixing
Remove grunt-complexity

The report itself is more or less useful because it detects stuff like
big objects (e.g. CSSProperty) as being too complicated. Furthermore, afaik
nobody refactors the code based on what the report says =).
Add clearer invariant in processUpdates

The `updatedChildren[j].parentNode.removeChild(updatedChildren[j]);` line below can fail if (1) we're moving/moving the same node twice or (2) the node we're looking for is gone completely. This makes it easier to distinguish between the two cases.

Perf shouldn't be a concern here because this is DOM code and invariants are fast in comparison.

Test Plan: grunt test
Merge pull request #1419 from spicyj/inv-process

Add clearer invariant in processUpdates
Merge pull request #1188 from spicyj/comp-spec-inv

Add clearer invariants to composite spec mixing