Low-level changes for `iron-list` integration (`fire` & `modelForElement`)

[kevinpschaaf]

Need two changes for iron-list integration:

• Polymer.Bind lib has a layering violation (noted TODO) where it calls this.fire from Bind. Should be changed to call Polymer.Base.fire, so that the model API decoupled (iron-list prepares the vanilla template as a bind model, which is not a Polymer.Base extension)
• Since Templatizer performs the _templateInstance decoration of top-level stamped children, modelForElement should be a Templatizer API since it relies on this implementation detail.