In shady dom + observeNodes, the disconnect throws

[notwaldorf]

If I use observeNodes to observe a local child, the disconnect bit throws:

[mkazlauskas]

Here are some hopefully helpful insights on iron-input:

• It throws because observeNodes triggers detached, I believe it shouldn't
• It doesn't throw if you move observeNodes from attached to ready callback
• It doesn't throw if you don't use a container like paper-input-container, or if that container doesn't have a <template>

[sorvell]

Fixed via #4112.