Fix templatizer under the template polyfill. forceDocumentUpgrade

[garlicnation]

was interacting problematically with un-prepped HTMLTemplateElements.
By forcing a bootstrap before the document upgrade, we sidestep this issue.

[dfreedm]

We should be able to remove this code then: https://github.com/Polymer/polymer/blob/master/src/mini/template.html#L36-L42

[dfreedm]

LGTM with nit to remove the leftover template bootstrap code in _prepTemplate

[dfreedm]

Notes for reasoning:

The HTMLTemplateElement polyfill will not run until DOMContentLoaded.
The HTMLImports Polyfill works around this by upgrading templates in each document before asking the CustomElements polyfill to observe the document and allow for element upgrades.

However, when using vulcanize with all the inlining flags, all elements will upgrade before the template polyfill runs, because of the call to forceDocumentUpgrade in the element upgrade flow. This can break dom-repeat and a few other elements that expect their template to have a content document.

This patch ensures that all templates in the document will be upgraded before all of the elements upgrade, just as in the standard use with HTMLImports.

@sorvell @kevinpschaaf PTAL

[kevinpschaaf]

Reviewed along with @sorvell.

Making the fix in dom-module's forceDocumentUpgrade isn't the best place, since ideally elements that are known to not have dom-module's (like dom-repeat) wouldn't try to look for a dom-module in the first place, fail, and call that function.

I'm inclined to instead add specific code to bootstrap template extension elements in Polymer's startup flow, ala add this to _prepTemplate (leaving the existing code as-is):

if (this.localName == 'template') {
  HTMLTemplateElement.decorate(this);       
  HTMLTemplateElement.bootstrap(this.content);      
}

This issue highlights a potentially serious performance issue with the way forceDocumentUpgrade does an expensive tree walk each time an element that intentionally has no dom-module is registered; opened separate issue for that here: #2708

[kevinpschaaf]

Per discussion, consider moving decorate & bootstrap code to CustomElements polyfill: https://github.com/webcomponents/webcomponentsjs/blob/master/src/CustomElements/upgrade.js#L60

[dfreedm]

I'm not sure I follow. Do you want the Template polyfill to actually use the CustomElement machinery?

[dfreedm]

Just to be 100% clear, this PR should be closed and we will proceed with webcomponents/webcomponentsjs#439 right?

[kevinpschaaf]

Yes, that's the plan.