elements-sk are difficult to test with karma

[jcgregorio]

The elements-sk are difficult to test with karma, see for example: karma-runner/karma#412

The problem is that you can't undefine an element. The solution is to use our own function that only calls window.customElements.define() if the tag hasn't yet been defined. That has the possibility of covering up real errors, such as two different definitions for the same tag name, so a console.log error message should be generated on each such conflict, but define() should never throw in the case of a conflict.