Minimum viable imperative distribution

imperative.js

var shadow = host.createShadowRoot({
  // called synchronously for each node *added* to shadow's distribution pool
  // called sequentially for each content in shadow until `true` is returned.
  shouldDistributeNodeToInsertionPoint: function(node, content) {
    // to implement catch-all
    return true;
    // to implement <content select="...">
    // return node.matches(content.getAttribute('select'));
    // to implement <content slot="...">
    // return node.getAttribute('slot') === content.getAttribute('slot');
  }
});

isolated.js

// when the shadowRoot is two-way isolated, the callback gets read only proxy elements instead of real ones.
var shadow = host.createShadowRoot({
  // called synchronously for each node *added* to shadow's distribution pool
  // called sequentially for each content in shadow until `true` is returned.
  shouldDistributeNodeToInsertionPoint: function(nodeProxy, contentProxy) {
    // to implement catch-all
    return true;
    // to implement <content select="...">
    // return nodeProxy.matches(contentProxy.getAttribute('select'));
    // to implement <content slot="...">
    // return nodeProxy.getAttribute('slot') === contentProxy.getAttribute('slot');
  }
});