Don't transfer `children` in `transferPropsTo`

06cff60bc1eec13ff804af9f74b3b2b37b5490b3 made it so that `this.props.children` was no longer set when
none were provided.

  var x = <div />;

This caused an issue if the code was relying on the following not
transferring children.

  return this.transferPropsTo(<div />);
  // this now transfer children
Better warnings for missing keys on arrays

We have less dynamic arrays in the code base now so let's start warning for all
the cases where we pass dynamic arrays without keys.

I use the displayName to point out which component's render method was
responsible. I only warn once per component. If the child was created in a
different component (and passed as a property) I also show the owner of the
child. Maybe it should've attached the key at a higher level.

This does give false positives for arrays that are truly static. Those should
probably be refactored to use the XML syntax if possible.