src/generators/dom/visitors/Element/Binding.js/getBindingGroup
function getBindingGroup ( generator, keypath ) {
	// TODO handle contextual bindings — `keypath` should include unique ID of
	// each block that provides context
	let index = generator.bindingGroups.indexOf( keypath );
	if ( index === -1 ) {
		index = generator.bindingGroups.length;
		generator.bindingGroups.push( keypath );
	}

	return index;
}