Address polymer-elements that take child elements

[jyasskin]

The documentation has a lot of examples of polymer-elements with attributes, but aside from mentioning that child elements are possible at http://www.html5rocks.com/en/tutorials/webcomponents/customelements/, I don't see anything about how to use the templating engine to generate different shadow-dom based on the child elements of the polymer-element.

For example, maybe I want to implement the dateRangePicker from http://www.html5rocks.com/en/tutorials/webcomponents/shadowdom/#toc-projection. Or maybe I want to generate a shadow-table whose rows depend on which child elements are present, but then use <content select> to attach those child elements.

[sorvell]

The simplest way to control what is rendered in a shadowRoot via templating is to use <template if>.

Note that a shadowRoot is generated for each element form which the element extends. To customize this behavior and control what's stamped into the element's shadowRoot, you can implement parseDeclarations and call this.shadowFromTemplate(template).