Re: [webcomponents] Custom elements disconnected from a document should not be upgraded (#419)

If being connected to a document was too restrictive, we could add a new flag on element to indicate whether an element is upgradable, and make all elements created by the parser to have that flag turned on by default.  e.g. `document.createElement('my-element', {upgrade: true})`.  We could also add a helper function on template element e.g. `template.instantiate()`, which does `template.content.cloneNode(true)` and then sets that flag on every single cloned node.

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