gistfile1.html

ES6:
<element name="g-comp">
    <template>
        ...
    </template>
    <script>
        // 'elementElement' would have to expose this.template
        var template = this.template;
        class Comp {
            constructor: function() {
                // super refers to Dimitri's generated constructor
                super();
                var root = this.webkitCreateShadowRoot();
                root.appendChild(template.content.cloneNode(true));
            }
        };
        this.lifecycle(Comp);
    </script>
</element>

pre-ES6:
<element name="g-comp">
    <template>
        ...
    </template>
    <script>
        // 'elementElement' would have to expose this.template
        var template = this.template;
        Comp = function() {
            // 'super'
            Comp.prototype.constructor();
            var root = this.webkitCreateShadowRoot();
            root.appendChild(template.content.cloneNode(true));
        };
        this.lifecycle(Comp);
    </script>
</element>

// 'elementElement' would have to expose this.template

Why? We can do firstElementChild or some other mechanism. No need to tightly couple it.

// 'super'
Comp.prototype.constructor();

That looks wrong. Comp.prototype.constructor === Comp. For ES3 we need to hard code the reference to the super constructor. (Also, you lost this in that call.)